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Arrow functions don’t have access to the arguments object as regular functions

While with a normal function you can use the arguments implicit object to see the given params:

function myFunction() {
    console.log(arguments);
}
myFunction('a', 'b');
// logs { 0: 'a', 1: 'b', length: 2 }

In an arrow function this will end up with an unexpected result:

function myFunction() {
    const arrowF = () => console.log(arguments);
    arrowF('c', 'd');
}

myRegularFunction('a', 'b'); 
// will log { 0: 'a', 1: 'b', length: 2 } 
// insead of the expected { 0: 'c', 1: 'd', length: 2 }

This happens because of the fact that that arrow functions don't have an arguments object. They also don't have their own this context and therefore the arguments is the one from the myRegularFunction context.

However, we can use a rest parameter to spread the arguments in an arrow function:

function myRegularFunction() {
    const arrowF = (...args) => console.log(args); 
    arrowF('c', 'd');
}
myRegularFunction('a', 'b'); 
// logs ['c', 'd']

And we can even make some cool stuff like combining regular parameters with a rest parameter:

function multiplyTheSum(multiplier, ...numbers) {
    console.log(multiplier); // 2
    console.log(numbers);    // [10, 20, 30]
    const sum = numbers.reduce((s, n) => s + n);
    return multiplier * sum;
}
multiplyTheSum(2, 10, 20, 30); // => 120

Btw, if you are interested you can read more also about the function declarations vs the function expressions.

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