Immutable Strings And Mutable Arrays
Strings in JavaScript are immutable which means that once a string is defined, it cannot be changed anymore. Trying to change a string with a standard String-function will only create a new string and not affect the original string. Consider the following example, where the value in string1 is not changed:
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var string1 = "LOREM IPSUM";
string1.toLowerCase();
console.log(string1);
On the other hand not all defined Array-functions are creating a new array instance, but they rather change and modify the original instance of an array, thus making the array mutable:
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// pop(): Removes the last element from an array:
var words = ["lorem", "ipsum"];
words.pop();
console.log(words) // words contains only one element
// push(): Adds a new element at the end of an array:
words = ["lorem", "ipsum"];
words.push("dolor");
console.log(words) // words contains three elements
// reverse(): reverses the elements of an array:
words = ["lorem", "ipsum", "dolor"];
words.reverse();
console.log(words) // the values of the array are reversed
// shift(): removes the first element of an array:
words = ["lorem", "ipsum", "dolor"];
words.shift();
console.log(words) // word contains two values
// sort(): sort an array alphabetically:
words = ["lorem", "ipsum", "dolor"];
words.sort();
console.log(words) // ["dolor", "ipsum", "lorem"]
// splice(): adds multiple items into an array:
words = ["lorem"];
words.splice(1, 0, "ipsum", "dolor");
console.log(words) // ["lorem", "ipsum", "dolor"]
// unshift(): adds a new element at the beggining of an array:
words = ["lorem", "ipsum"];
words.unshift("dolor");
console.log(words) // ["dolor", "lorem", "ipsum"]
You can find the code of the article here.