What is the Rest Parameter in a Function?

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A JavaScript function can take any number of parameters. Unlike other languages like C# and Java, you can pass any number of parameters while calling a JavaScript function. JavaScript functions allows unknown number of function parameters. Before ECMAScript 6, JavaScript had arguments variable to access these unknown or variable number of parameters, which is an array-like object but not an array. Consider this code listing to understand the argumentsvariable:

As you see, that arguments object is used to access unknown or variable function parameters.  Even though arguments uses length property and square brackets, it is not a real JavaScript array. You cannot use other JavaScript array methods like pop, push, slice etc. with the arguments object.  Some of the problems in using arguments are:

  • JavaScript function arguments object is not a real JavaScript array; therefore, you cannot use other array methods like pop, push, slice, etc. with it
  • It is difficult to access arguments object of outer function in an inner function. To use it, you need to assign outer function arguments object in a variable, and then use it in an inner function
  • If you want to use arguments object as an array, then you need to convert it manually using Aarry.prototype.slice

ECMAScript 6 introduces a new feature, Rest Parameters, which represents an unknown number of parameters as an array in a function. It not only represents extra parameters as an array, it also solves many of the problems of the arguments object.  Let us rewrite the above add function using the rest parameters.

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