How does Array.push() method work exactly in Javascript?

Question

while I was taking online CS class, the topic was about an overview of how Array and memory works, and the teacher used C as an Example as that you cannot simply add extra element to an existing Array while the Array is already full with element. As a beginner developer who started with JavaScript, even though I know JavaScript is a high level language and Array.push() is already a familiar function to me, but it seems like it doesn't fit such context, out of curiosity, I've search through google and StackOverflow, I just don't see people discussing why JavaScript can just add extra elements to an existing Array.

Does JavaScript simply create a new Array with added element and point the variable I've already assigned to to the new Array or something else?

• Gary: In JavaScript, only objects and arrays are mutable, not primitive values. ... A mutable object is an object whose state can be modified after it is created. This means the object is the same along with the memory reference. developer.mozilla.org/en-US/docs/Glossary/Mutable
• potatopeelings: developer.mozilla.org/en-US/docs/Web/JavaScript/… and stackoverflow.com/questions/12640216/…
• Gary: A array.push adds item to the end of the array C and a removal using pop() removes the item from the end of object C array developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Answer 1

Arrays in javascript are not the same as an array in C. In C you'll allocate a new array with type and a fixed size and if you want to alter the size you'll have to create a new one. For javascript arrays on the other hand if you have a look at the description of arrays over at MDN you'll see this line.

Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Neither the length of a JavaScript array nor the types of its elements are fixed. Since an array's length can change at any time, and data can be stored at non-contiguous locations in the array

So while it is called "array" it is more like a list which lets you resize it freely (and also store anything, because javascript).

There is also another question about the semantics of the push/pop methods here on SO that can shine some more light on those: How does the Javascript Array Push code work internally

Answer 2

A JavaScript Array is not like a C array, it's more like a C++ std::vector. It grows in length by dynamically allocating memory when needed.

As stated in the reference documentation for std::vector:

Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. [...] Reallocations are usually costly operations in terms of performance.

• Darren Wang: thank you for the reply, does this mean that when using such methods in JavaScript, it will abandon the address of an old array and then assign a new address in memory with new array that I just changed?
• Patrick Roberts: @DarrenWang those details are not observable, but typically it allocates memory in exponentially larger increments. The idea is to try and minimize both the amount of unused memory and also the number of times it has to "abandon the old array" and copy the underlying data to the new address.