Tuesday, 2 April 2019
Set a default parameter value for a JavaScript function
Answer
Answer
I would like a JavaScript function to have optional arguments which I set a default on, which get used if the value isn't defined (and ignored if the value is passed). In Ruby you can do it like this:
def read_file(file, delete_after = false)
# code
end
Does this work in JavaScript?
function read_file(file, delete_after = false) {
// Code
}
Answer
From ES6/ES2015, default parameters are in the language specification.
function read_file(file, delete_after = false) {
// Code
}
just works.
Reference: Default Parameters - MDN
Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.
You can also simulate default named parameters via destructuring:
// the `= {}` below lets you call the function without any parameters
function myFor({ start = 5, end = 1, step = -1 } = {}) { // (A)
// Use the variables `start`, `end` and `step` here
···
}
Pre ES2015,
There are a lot of ways, but this is my preferred method — it lets you pass in anything you want, including false or null. (typeof null == "object")
function foo(a, b) {
a = typeof a !== 'undefined' ? a : 42;
b = typeof b !== 'undefined' ? b : 'default_b';
...
}
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