What is a potential pitfall with using typeof bar === "object" to determine if bar is an object? How can this pitfall be avoided?
What will the code below output to the console and why?
(function(){
var a = b = 3;
})();
console.log("a defined? " + (typeof a !== 'undefined'));
console.log("b defined? " + (typeof b !== 'undefined'));
View the answer →What will the code below output to the console and why?
var myObject = {
foo: "bar",
func: function() {
var self = this;
console.log("outer func: this.foo = " + this.foo);
console.log("outer func: self.foo = " + self.foo);
(function() {
console.log("inner func: this.foo = " + this.foo);
console.log("inner func: self.foo = " + self.foo);
}());
}
};
myObject.func();
View the answer →
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Hire Toptal’s JavaScript developersWhat is the significance of, and reason for, wrapping the entire content of a JavaScript source file in a function block?
View the answer →What is the significance, and what are the benefits, of including 'use strict' at the beginning of a JavaScript source file?
Consider the two functions below. Will they both return the same thing? Why or why not?
function foo1()
{
return {
bar: "hello"
};
}
function foo2()
{
return
{
bar: "hello"
};
}
View the answer →What is NaN? What is its type? How can you reliably test if a value is equal to NaN?
What will the code below output? Explain your answer.
console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 0.3);
View the answer →Discuss possible ways to write a function isInteger(x) that determines if x is an integer.
In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?
(function() {
console.log(1);
setTimeout(function(){console.log(2)}, 1000);
setTimeout(function(){console.log(3)}, 0);
console.log(4);
})();
View the answer →Write a simple function (less than 160 characters) that returns a boolean indicating whether or not a string is a palindrome.
View the answer →Write a sum method which will work properly when invoked using either syntax below.
console.log(sum(2,3)); // Outputs 5
console.log(sum(2)(3)); // Outputs 5
View the answer →Consider the following code snippet:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function(){ console.log(i); });
document.body.appendChild(btn);
}
(a) What gets logged to the console when the user clicks on “Button 4” and why?
(b) Provide one or more alternate implementations that will work as expected.
View the answer →Assuming d is an “empty” object in scope, say:
var d = {};
…what is accomplished using the following code?
[ 'zebra', 'horse' ].forEach(function(k) {
d[k] = undefined;
});
View the answer →What will the code below output to the console and why?
var arr1 = "john".split('');
var arr2 = arr1.reverse();
var arr3 = "jones".split('');
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));
View the answer →What will the code below output to the console and why ?
console.log(1 + "2" + "2");
console.log(1 + +"2" + "2");
console.log(1 + -"1" + "2");
console.log(+"1" + "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);
View the answer →The following recursive code will cause a stack overflow if the array list is too large. How can you fix this and still retain the recursive pattern?
var list = readHugeList();
var nextListItem = function() {
var item = list.pop();
if (item) {
// process the list item...
nextListItem();
}
};
View the answer →What is a “closure” in JavaScript? Provide an example.
View the answer →What will be the output of the following code:
for (var i = 0; i < 5; i++) {
setTimeout(function() { console.log(i); }, i * 1000 );
}
Explain your answer. How could the use of closures help here?
View the answer →What would the following lines of code output to the console?
console.log("0 || 1 = "+(0 || 1));
console.log("1 || 2 = "+(1 || 2));
console.log("0 && 1 = "+(0 && 1));
console.log("1 && 2 = "+(1 && 2));
Explain your answer.
View the answer →What will be the output when the following code is executed? Explain.
console.log(false == '0')
console.log(false === '0')
View the answer →What is the output out of the following code? Explain your answer.
var a={},
b={key:'b'},
c={key:'c'};
a[b]=123;
a[c]=456;
console.log(a[b]);
View the answer →What will the following code output to the console:
console.log((function f(n){return ((n > 1) ? n * f(n-1) : n)})(10));
Explain your answer.
View the answer →Consider the code snippet below. What will the console output be and why?
(function(x) {
return (function(y) {
console.log(x);
})(2)
})(1);
View the answer →What will the following code output to the console and why:
var hero = {
_name: 'John Doe',
getSecretIdentity: function (){
return this._name;
}
};
var stoleSecretIdentity = hero.getSecretIdentity;
console.log(stoleSecretIdentity());
console.log(hero.getSecretIdentity());
What is the issue with this code and how can it be fixed.
View the answer →Create a function that, given a DOM Element on the page, will visit the element itself and all of its descendents (not just its immediate children). For each element visited, the function should pass that element to a provided callback function.
The arguments to the function should be:
- a DOM element
- a callback function (that takes a DOM element as its argument)
Testing your this knowledge in JavaScript: What is the output of the following code?
var length = 10;
function fn() {
console.log(this.length);
}
var obj = {
length: 5,
method: function(fn) {
fn();
arguments[0]();
}
};
obj.method(fn, 1);
View the answer →Consider the following code. What will the output be, and why?
(function () {
try {
throw new Error();
} catch (x) {
var x = 1, y = 2;
console.log(x);
}
console.log(x);
console.log(y);
})();
View the answer →What will be the output of this code?
var x = 21;
var girl = function () {
console.log(x);
var x = 20;
};
girl ();
View the answer →How do you clone an object?
View the answer →for (let i = 0; i < 5; i++) {
setTimeout(function() { console.log(i); }, i * 1000 );
}
What will this code print?
View the answer →What do the following lines output, and why?
console.log(1 < 2 < 3);
console.log(3 > 2 > 1);
View the answer →How do you add an element at the begining of an array? How do you add one at the end?
View the answer →Imagine you have this code:
var a = [1, 2, 3];
a) Will this result in a crash?
a[10] = 99;
b) What will this output?
console.log(a[6]);
View the answer →What is the value of typeof undefined == typeof NULL?
What would following code return?
console.log(typeof typeof 1);
View the answer →What will the following code output and why?
var b = 1;
function outer(){
var b = 2
function inner(){
b++;
var b = 3;
console.log(b)
}
inner();
}
outer();
View the answer →* There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer
them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work.
