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Use Default Parameters

Default parameters allow providing default values for function parameters. These default values are used if nothing or undefined is passed into the function. For example, function f(p = 42) { ... } sets p to 42 if no value is passed in.

Previous ways of setting default values, for example p = p || 42 can be converted to default parameters. This refactoring supports converting the following expressions and variants of them:

  • or assignment, e.g. p = p || 42 or p ||= 42
  • nullish coalescing operator, e.g. p = p ?? 42
  • ternary with falsy check on parameter, e.g. p = p ? p : 42
  • ternary with null check, e.g. p = p == null ? 42 : p or p = p != null ? p : 42
  • ternary with undefined check, e.g. p = p === undefined ? 42 : p
  • ternary with typeof check, e.g. p = typeof p === "undefined" ? 42 : p

Learn More: Default Parameters (MDN)

Default values are only used in place of undefined. The code that is replaced by this refactoring may have provided defaults for falsy or null values as well. This may lead to undesired behavior, for example:

function example(x) {
    x = x == null ? "default" : x;
    console.log(x);
}

example(null); // prints "default"

becomes

function example(x = "default") {
    console.log(x);
}

example(null); // prints "null"

Original Code

Changes

1function x(p) {1function x(p = 42) {
2 p = p || 42;
3 console.log(p);2 console.log(p);
4}3}
1function x(p) {
2 p = p || 42;
1function x(p = 42) {
32 console.log(p);
43}

Transformed Code

function x(p = 42) {
  console.log(p);
}

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