Note: Keep in mind that many of these questions are open-ended and could lead to interesting discussions that tell you more about the person's capabilities than a straight answer would.
⭐ Junior
Answer
A function's this keyword behaves a little differently in JavaScript compared to other languages. It also has some differences between strict mode and non-strict mode.
In the global execution context (outside of any function), this refers to the global object whether in strict mode or not.
Inside a function, the value of this depends on how the function is called.
Explain arrow functions this, bind method
As an object method its this is set to the object the method is called on.
⭐ Junior
Answer
const is a signal that the identifier won’t be reassigned. It needs initialisation upfront, so you can't write const something;let, is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm.
var is now the weakest signal available when you define a variable in JavaScript. The variable may or may not be reassigned, and the variable may or may not be used for an entire function, or just for the purpose of a block or loop. It's declaration is hoisted, instead of let and const.
for ( var i=0; i<2; i++ ) {} console.log(i) // exists outside the blockscope
for ( let i=0; i<2; i++ ) {} console.log(i) // only exists inside the blockscope
for ( const i=0; i<2; i++ ) {} console.log(i) // error reassignment, but only on top-level
for ( const cnt={i:0}; cnt.i<2; cnt.i++ ) {} // only exists inside the blockscope
⭐ Junior
Answer
This is the strict comparision operator e.g. 5 == '5' = true vs 5 === '5' = false, this means that it checks the value and also the type, so that Int 5 isn't equal a Str 5.
⭐ Junior
Answer
Access one element:
let byID = document.getElementById('id');
let qS = document.querySelector('#id');
They return the first matching node. querySelector is the new selector interface, should be faster, but depends on browser implementation. querySelector can take any css-selector and is more comfortable.
Access one and more:
let byClass = document.getElementsByClassName(classname);
let qSA = document.querySelectorAll('.classname');
They return a non-live NodeList, which is an array-like list of elements, array-like means that some functions are missing like push(), pop()).
⭐ Junior
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⭐ Junior
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q-unit, mocha, chai, sinonJS, jasmine, ...
⭐ Junior
Answer
Means that the declaration moved to the top of the current scope (current script or the current function). JavaScript only hoists declarations, not initializations.
let and const don't get hoisted.
⭐ Junior
Answer
Switches to strict mode which helps to prevent common errors like using unsafe operators
⭐ Senior
Answer
bind is a method to bind the current context for later execution e.g.
element.addEventListener('click', this.onClick.bind(this), false);
it creates a new function which prevents accidental loss of scope. An alternative approach is to use apply, call or ES6 fat-arrow function.
⭐ Senior
Answer
Function that will take a function as argument or return a new function. For example [].map/filter/reduce are higer order functions.
⭐ Senior
Answer
map - to iterate over an array and return a new one filter - to filter an array and return a new filtered one reduce - takes and reducer function which evaluate against every element and can produce every desired output (filter, map or simple value like sum)
⭐ Senior
Answer
If you have to watch a lot of elements and performance is key