I want to supply a function with only some of its parameters, and let the rest revert to their defaults. For example, I have a function like this:
function doSomething(a = 'apple', b, c){
//Do something with a, b and c
}
Now I want to call this function, but only supply arguments b and c (so that 'a' defaults to 'apple'). What is the best way to do this?
Normally, the defaults should be at the very end of your function (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters):
function doSomething(b, c, a = 'apple'){
//Do something with a, b and c
}
There is no absolute best way of doing it. But, there are few ways that can simplify and bring in more meaning to the code written with default parameters.
You can define all the default parameter in the starting of the arguments and for rest of the parameters which doesn't have a default value, make use of the rest operator.
Eg -
function doSomething(a='apple',b='banana',...args){
console.log(a);
console.log(b);
console.log(args);
}
let x,y;
doSomething(x,y,"hello","hey","hi");
output -
Go through the following link to get more understanding of the rest operator.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters
Though a little unrelated. By default javascript, parameters are undefined if they are not passed any value or reference. You can check that the parameters are not undefined inside your code block.
A useful pattern (used widely in the JavaScript world) is to allow callers to pass an options object, if entries are missing they will fall back to default values.
It's particularly useful if you're dealing with a lot of parameters.
function doSomething(a, b, options) {
// Specify your defaults here...
let defaults = { c: 3, d: "bar" };
options = Object.assign({}, defaults, options);
let { c, d } = options;
console.log("doSomething: (a,b,c,d):", a, b, c, d);
}
doSomething(1,2, { c: 5, d: "foo" });
doSomething(1,2);
hope this will be clear if value of a param should always be same :
function doSomething(b, c, a = 'apple'){
console.log({b:b},{c:c},{a:a})
}
doSomething('hi','hello','a is not apple here--')
function doSomething1(b, c, a){
a = 'apple'
console.log({b:b},{c:c},{a:a})
}
doSomething1('hi','hello','a is apple here--')