I don’t understand why we use “spread operator” in this example. I
I mean why we don’t just use arr2 = arr1?
Also, I have an another question, what does .apply() do?
I really tried to understand it but failed its purpose. Your code so far
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
let arr2;
(function() {
"use strict";
arr2 = [...arr1]; // change this line
})();
console.log(arr2);
Your browser information:
User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0.
Spread syntax allows an iterate such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected, or an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected.
If you want to need more understand about spread operator just go here
2- What does .apply() do? apply is very similar to call(), except for the type of arguments it supports. You use an arguments array instead of a list of arguments (parameters). With apply, you can also use an array literal, for example, func.apply(this, ['eat', 'bananas']), or an Array object, for example, func.apply(this, new Array('eat', 'bananas')).
If you want to need more understand about .apply() just gohere
Hope this will help you, good luck my friend and have a happy coding.
In javascript values are assigned by reference or value. When you assign an object like an array, you’re just adding a reference. When you use the spread operator, you create a new copy
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
const arr2 = arr1 // assigned the reference to the same array
console.log(arr1 === arr2) // true
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
const arr2 = [...arr1] // assigned to a copy of arr1 values
console.log(arr1 === arr2) // false
Because I wasn’t clear enough. Let me try again and split it up this time
When you assign a variable to another variable, it will be passed as a reference or as the value. This is a more complex topic, so let’s just agree that assigning a variable to an array only passes the reference. That means
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
const arr2 = arr1 // assigned the reference to the same array
console.log(arr1 === arr2) // true
Here what we are saying is that arr2 is equal to arr1. Not that the values are equal, but that the actual arrays are the same. Try this in the console and you’ll see they are in fact the same
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
const arr2 = arr1 // assigned the reference to the same array
console.log(arr1 === arr2) // true
arr1[0] = 'I was mutated!'
console.log(arr1[0]) // I was mutated!
console.log(arr2[0]) // I was mutated! -- oh no, arr2 was changed also!
Now the spread operator returns a new copy, with all the same values of the original
So let’s try that out
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
const arr2 = [...arr1] // assigned to a copy of arr1 values
console.log(arr1 === arr2) // false
arr1[0] = 'I was mutated!'
console.log(arr1[0]) // I was mutated!
console.log(arr2[0]) // JAN -- yay arr2 has not changed!
Tell me if something is still not clear and I’ll try to find another method.
It seems that I had the wrong idea of “assigning means giving the values” while “assigning means a reference to the first, so if the first has changed, the assigned var will be changed too”
But I still didn’t understand what does apply() do, why we don’t just use math.max.apply(), why we don’t just write math.max alone? What is apply()'s purpose?
Sorry for the many questions. I really have a hard time understanding these new lessons.
This is something even experienced js devs can get wrong, so don’t feel bad about it.
First some term clarification:
function parameters
what the function accepts
function arguments
what the function gets sent
Math.max only accepts a a group of numbers as parameters. So
// this works because arguments are properly separated
Math.max(1,2,3) // 3
/// But this doesn't because you're only sending 1 array as an argument
Math.max([1,2,3]) // NaN
What .apply() does is that it allows you pass in arguments as an array. Internally .apply() will extract all the items in the array into individual arguments.
This allows you to use Math.max with an array of numbers.
Don’t be sorry. Everyone is a beginner starting out. I’m just glad my explanations work for you.
Just wanted to say thank you so much for taking the time to post explanations on this forum. I’ve seen your posts crop up every so often and they’re always so clear. Trouble with other forums sometimes is that the explanations rely on a knowledge of other terminology that beginners may not have yet. So yeah, many thanks!
You’re welcome! This is a very nice compliment. I suffered through the same issues when learning to code myself. So it’s nice to know that others find my breaking things down to be helpful.
The apply method exists on Function.prototype. Meaning that it’s available on all functions. Therefore the method does not know the this context that is calling it, and you have to supply it as the first argument. By passing null, you’re essentially saying that the this context is irrelevant.
