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7.8.8 unshift() and shift()

stack.push(3); // stack: [1,3] Returns 2 stack.pop(); // stack: [1] Returns 3 stack.push([4,5]); // stack: [1,[4,5]] Returns 2 stack.pop() // stack: [1] Returns [4,5] stack.pop(); // stack: [] Returns 1


7.8.8 unshift() and shift()

The unshift() and shift() methods behave much like push() and pop(), except that they insert and remove elements from the beginning of an array rather than from the end. unshift() adds an element or elements to the beginning of the array, shifts the existing array elements up to higher indexes to make room, and returns the new length of the array. shift() removes and returns the first element of the array, shifting all subsequent elements down one place to occupy the newly vacant space at the start of the array. For example:

var a = []; // a:[] a.unshift(1); // a:[1] Returns: 1 a.unshift(22); // a:[22,1] Returns: 2 a.shift(); // a:[1] Returns: 22 a.unshift(3,[4,5]); // a:[3,[4,5],1] Returns: 3 a.shift(); // a:[[4,5],1] Returns: 3 a.shift(); // a:[1] Returns: [4,5] a.shift(); // a:[] Returns: 1

Note the possibly surprising behavior of unshift() when it’s invoked with multiple arguments. Instead of being inserted into the array one at a time, arguments are inserted all at once (as with the splice() method). This means that they appear in the resulting array in the same order in which they appeared in the argument list. Had the elements been inserted one at a time, their order would have been reversed.

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