Array.splice() Unexpected Behavior When Index Is Undefined

Feb 15, 2013 • 1 min read

Today I discovered something you should know about Array.splice() in Javascript. According to this page splice() has a first parameter of index which is supposed to be a number representing the index of the array you wish to splice at. Well, apparently when this index is undefined it acts as though you passed 0 instead.

// Create a sample array
var array = [ 'a', 'b', 'c' ];

console.log(array);

// Remove 1 element from index undefined
array.splice(undefined, 1);

console.log(array);

/**
 * OUTPUT
 *
 * [ 'a', 'b', 'c' ]
 * [ 'b', 'c' ]
 */

Keep this in mind next time you’re using Array.splice() as it is very unexpected behaviour.

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