React Native Interview Handbook — Part 8 of 10: Code Output Challenges

A developer published Part 8 of a 10-part React Native Interview Handbook series, featuring 70 code-output challenges. The challenges cover JavaScript concepts such as variable scoping, closures, object references, and array methods, with explanations of the underlying language rules.

This is Part 8 of 10 , a bonus practice article with 70 code-output challenges . Each challenge asks you to predict the result before revealing the answer and reasoning. This Dev.to series has five core handbook articles plus five focused practice extras. Open the series page to move through the complete reading order: Read the code, state the exact output or error, then explain the language rule. Do not run the snippet until you have committed to an answer. For React Native interviews, connect the JavaScript behavior to rendering, state updates, list handling, or the JavaScript thread when relevant. this async / await , and microtasksPredict the exact output before opening the answer. js let total = 0; for let i = 0; i < 3; i++ { total += i; } console.log total ; Why: The loop adds 0, 1, and 2. var callback loop Predict the exact output before opening the answer. js for var i = 0; i < 3; i++ { setTimeout = { console.log i ; }, 0 ; } Why: var creates one shared function-scoped binding. let callback loop Predict the exact output before opening the answer. js for let i = 0; i < 3; i++ { setTimeout = { console.log i ; }, 0 ; } Why: let creates a fresh binding for each iteration. Predict the exact output before opening the answer. js const a = 1, 2 ; const b = a; b.push 3 ; console.log a.length ; Why: a and b reference the same array. Predict the exact output before opening the answer. js const a = { user: { name: 'A' } }; const b = { ...a }; b.user.name = 'B'; console.log a.user.name ; Why: Object spread copies only the outer object. map with a missing return Predict the exact output before opening the answer. js console.log 1, 2 .map x = { x 2; } , ; Why: A block-bodied arrow function needs an explicit return . reduce accumulator Predict the exact output before opening the answer. js console.log 1, 2, 3 .reduce sum, x = sum + x, 0 ; Why: The accumulator starts at zero and receives every value. Predict the exact output before opening the answer. console.log 10, 2, 1 .sort ; Why: Without a comparator, values are sorted as strings. Predict the exact output before opening the answer. js const a = 1, , 3 ; console.log a.length, 1 in a ; Why: The missing element is a hole, but array length remains three. filter Boolean Predict the exact output before opening the answer. console.log 0, 1, '', 2, null .filter Boolean ; Why: Boolean removes every falsy value. Predict the exact output before opening the answer. console.log 0 == false, 0 === false ; Why: Loose equality coerces types; strict equality does not. Predict the exact output before opening the answer. console.log 0 || 10, 0 ?? 10 ; Why: || falls back for falsy values, while ?? only falls back for nullish values. Predict the exact output before opening the answer. js const user = null; console.log user?.profile?.name ?? 'Guest' ; Why: Optional chaining returns undefined , then ?? supplies the fallback. Predict the exact output before opening the answer. js function make { let n = 0; return = ++n; } const next = make ; console.log next , next ; Why: The returned function retains its lexical n binding. this Predict the exact output before opening the answer. js const user = { name: 'A', show { return = this.name ; }, }; console.log user.show ; Why: The arrow captures this from the regular show method. Predict the exact output before opening the answer. js function show { return this.name; } const f = show.bind { name: 'A' } ; console.log f ; Why: bind creates a function with a fixed receiver. Predict the exact output before opening the answer. js console.log 'A' ; setTimeout = console.log 'B' , 0 ; Promise.resolve .then = console.log 'C' ; console.log 'D' ; Why: Synchronous work runs first, then microtasks, then timer tasks. await continuation Predict the exact output before opening the answer. async function run { console.log 1 ; await 0; console.log 2 ; } run ; console.log 3 ; Why: Code after await continues in a microtask. Predict the exact output before opening the answer. js Promise.reject 'x' .catch = 2 .then console.log ; Why: Returning from catch fulfills the next promise. map result Predict the exact output before opening the answer. js const x = 1, 2 .map async n = n 2 ; console.log x 0 instanceof Promise ; Why: An async callback always returns a Promise. Predict the exact output before opening the answer. const a = 1, b = 2 = undefined, null ; console.log a, b ; Why: Defaults apply to undefined , not null . Predict the exact output before opening the answer. js const o = {}, a = {}, b = {}; o a = 'one'; o b = 'two'; console.log o a ; Why: Plain-object keys are coerced to the same string. Predict the exact output before opening the answer. js const parent = { role: 'admin' }; const user = Object.create parent ; console.log user.role ; Why: Property lookup follows the prototype chain. Predict the exact output before opening the answer. js const p = { x: 1 }, o = Object.create p ; o.x = 2; delete o.x; console.log o.x ; Why: Deleting the own property reveals the inherited one. Object.freeze is shallow Predict the exact output before opening the answer. js const o = Object.freeze { x: { n: 1 } } ; o.x.n = 2; console.log o.x.n ; Why: The nested object is not frozen. Predict the exact output before opening the answer. js let count = 0; const setCount = v = { count = v; }; setCount count + 1 ; setCount count + 1 ; console.log count ; Why: This plain JavaScript model evaluates each update immediately; React batching differs, so discuss that distinction in an interview. Predict the exact output before opening the answer. js let id; const debounce = f = x = { clearTimeout id ; id = setTimeout = f x , 0 ; }; const f = debounce console.log ; f 1 ; f 2 ; Why: The second call clears the first pending timer. Promise.all result order Predict the exact output before opening the answer. Promise.all Promise.resolve 2 , 1 .then console.log ; Why: Promise.all preserves input order after resolving values. Predict the exact output before opening the answer. console.log add 1, 2 ; function add a, b { return a + b; } Why: Function declarations are initialized during scope creation. Predict the exact output before opening the answer. js const a = { x: undefined }; const b = JSON.parse JSON.stringify a ; console.log 'x' in b ; Why: JSON serialization drops undefined object properties. These 40 additional challenges focus on the JavaScript traps frequently used in interview rounds: hoisting, scope, == , === , arrays, objects, coercion, and reference identity. var value Predict the exact output or error before opening the answer. js console.log score ; var score = 10; Why: var declarations are initialized as undefined before execution; the assignment happens later. let Predict the exact output or error before opening the answer. js console.log score ; let score = 10; Why: let exists in the Temporal Dead Zone until its declaration runs. Predict the exact output or error before opening the answer. show ; function show { console.log 'ready' ; } Why: Function declarations are initialized with their function body during scope creation. Predict the exact output or error before opening the answer. js show ; var show = function { console.log 'ready' ; }; Why: Only show is hoisted as undefined ; it is not callable before assignment. var shadows outer state Predict the exact output or error before opening the answer. js var value = 'outer'; function printValue { console.log value ; var value = 'inner'; } printValue ; Why: The local var value is hoisted and shadows the outer variable. const Predict the exact output or error before opening the answer. js { const token = 'secret'; } console.log typeof token ; Why: const is block-scoped; typeof on an undeclared name returns undefined . var binding Predict the exact output or error before opening the answer. js function update value { console.log value ; var value = 'new'; console.log value ; } update 'old' ; Why: A parameter and a same-named var share one function-scoped binding. Predict the exact output or error before opening the answer. js let value = 1; function read value = value { return value; } console.log read ; Why: The parameter binding shadows the outer name while it is still uninitialized. Predict the exact output or error before opening the answer. js const user = new User ; class User {} Why: Classes are not usable before their declaration is evaluated. Predict the exact output or error before opening the answer. js const run = function internal { return typeof internal; }; console.log run ; console.log typeof internal ; Why: The expression name is available inside the function body, not outside it. var leaks from an if block Predict the exact output or error before opening the answer. js if true { var message = 'visible'; } console.log message ; Why: var has function scope rather than block scope. let does not leak from an if block Predict the exact output or error before opening the answer. js if true { let message = 'hidden'; } console.log typeof message ; Why: let remains inside the block. Predict the exact output or error before opening the answer. js let count = 1; { let count = 2; console.log count ; } console.log count ; Why: The inner block creates a separate binding. Predict the exact output or error before opening the answer. js let status = 'loading'; const readStatus = = console.log status ; status = 'ready'; readStatus ; Why: Closures retain the binding, not a copied old value. for loop let scope Predict the exact output or error before opening the answer. js for let index = 0; index < 1; index++ {} console.log typeof index ; Why: The loop initializer let does not escape the loop. for loop var scope Predict the exact output or error before opening the answer. js for var index = 0; index < 1; index++ {} console.log index ; Why: The var binding is available after the loop. let Predict the exact output or error before opening the answer. js let value = 1; let value = 2; Why: A lexical binding cannot be redeclared in the same scope. const Predict the exact output or error before opening the answer. js const value = 1; value = 2; Why: const prevents rebinding after initialization. Predict the exact output or error before opening the answer. js var name = 'first'; function read { console.log name ; var name = 'second'; } read ; Why: The function-local var name shadows the outer value before its assignment. Predict the exact output or error before opening the answer. js function getValue { return 1; } var getValue = 2; console.log typeof getValue ; Why: The later assignment replaces the function value at runtime. Predict the exact output or error before opening the answer. console.log === ; Why: Each array literal creates a different object. Predict the exact output or error before opening the answer. console.log == false ; Why: The empty array becomes an empty string, then zero; false also becomes zero. Predict the exact output or error before opening the answer. console.log == 0 ; Why: Loose equality converts the empty array to an empty string and then to zero. Predict the exact output or error before opening the answer. console.log == '' ; Why: An empty array converts to an empty string. Predict the exact output or error before opening the answer. console.log 0 == 0 ; Why: 0 converts to the string "0" , then to numeric zero. Predict the exact output or error before opening the answer. console.log 1 == true ; Why: 1 converts to "1" ; both sides then compare as numeric one. Predict the exact output or error before opening the answer. console.log 1, 2 == '1,2' ; Why: Arrays convert to their comma-joined string representation for this comparison. Predict the exact output or error before opening the answer. console.log == {} ; Why: The array becomes "" while the object becomes " object Object " . Predict the exact output or error before opening the answer. console.log {} === {} ; Why: Object equality compares identity, not matching properties. Predict the exact output or error before opening the answer. console.log null == undefined, null === undefined ; Why: Loose equality has a special null/undefined rule; strict equality requires identical types. Predict the exact output or error before opening the answer. console.log null == 0 ; Why: The null/undefined loose-equality exception does not include zero. Predict the exact output or error before opening the answer. console.log '' == 0, '' === 0 ; Why: Loose equality coerces the empty string to zero; strict equality does not coerce. Predict the exact output or error before opening the answer. console.log false == '0' ; Why: Both operands coerce to numeric zero during loose equality. NaN equality Predict the exact output or error before opening the answer. console.log NaN === NaN, Object.is NaN, NaN ; Why: NaN is not equal to itself with === ; Object.is handles it specially. Object.is and signed zero Predict the exact output or error before opening the answer. console.log 0 === -0, Object.is 0, -0 ; Why: === treats signed zeroes as equal; Object.is distinguishes them. Predict the exact output or error before opening the answer. console.log null == '' ; Why: A one-item array containing null converts to an empty string. Predict the exact output or error before opening the answer. console.log undefined == '' ; Why: A one-item array containing undefined also converts to an empty string. Predict the exact output or error before opening the answer. console.log Boolean new Boolean false ; Why: Wrapper objects are objects, and objects are truthy. Predict the exact output or error before opening the answer. console.log Boolean {} , Boolean ; Why: All ordinary objects and arrays are truthy, including empty ones. Predict the exact output or error before opening the answer. console.log '5' === 5, Number '5' === 5 ; Why: Strict equality compares both type and value; explicit conversion makes types match. Revisit Parts 6 and 7 for larger output-based and coding practice sets, then return to the core handbook for architecture, system design, and behavioral preparation.