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Welcome to the Javascript-use-of-apply- wiki!

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📕 The Javascript apply method is similar to the method call. The big difference is the ability of apply to take an array of different numbers of arguments:

function manyArguments(a,b,c,d)
{
    return a + b + c + d;
}

//Outputs 10
manyArguments.apply(this,[1,2,3,4]); 

This could be important for the use of variadic functions or also known as variable-arity functions. An example for a variadic function could be one which calculates the sum of a series of numbers:

//Variadic Function
function calculateSum(listOfNumbers)
{
    var sum = 0;
  
    for(var i = 0; i < listOfNumbers.length; i++)
    {
        sum += listOfNumbers[i];
    }
  
    return sum;
}

//Outputs 6
calculateSum([1,2,3]);

With the use of arguments and apply, the function calculateSum could be written and called like follows:

//Variadic Function - no parameter(s)
function calculateSum()
{
    var sum = 0;
  
    //"arguments" is an implicit local variable 
    //which handles the arguments of its function
    for(var i = 0; i < arguments.length; i++)
    {
        sum += arguments[i];
    }
  
    return sum;
}

//Outputs 6
calculateSum.apply(null, [1,2,3]);

📕 The null receiver in calculateSum.apply could also be replaced by an object, keyword this(see first code listing) or by primitive values.

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