[javascript] Setting default values for parameters of a function

In javascript, you can call a function with any number of parameters you like.

If the number of parameters you provided is less than the number of parameters specified in function declaration, then the rest of the parameters will be undefined.

Here’s an example:

// sayHello.js

function sayHello(name) {
    console.log('Hello, ' + name);
}

sayHello()                // Hello, undefined
sayHello('Brian');        // Hello, Brian

What happened here is that name, the undefined primitive type, is coerced into a string which contains 'undefined' value.
Then Hello, and 'undefined' are concatenated by + operator.

What if you want to specify a value when the parameter is undefined ? Well, You can use || operator.

// sayHello_default_value.js

function sayHello(name) {
    name = name || '<Your name>';    // line 4
    console.log('Hello, ' + name);
}

sayHello();            // Hello, <Your name>
sayHello('Brian');     // Hello, Brian

You can simply think that line 4 means if the value of name is undefined, then assign it with <Your name> string value.
However, there’s something you should be aware of with the above statement.

Let’s dig deeper with name = name || '<Your name>';

> node
> 0 || 1
1       
> 1 || 2
1       
> undefined || 'Brian'
'Brian'

From the above execution results, we can see that if || is passed with two values that can be coerced into true and false, or two values that can both be coerced into true, it returns the first value that coerced into true.

So what happened at line 4 in sayHello_default_value.js is that
name and '<Your name>' are first coerced into boolean values. Because '<Your name>' is a string with length larger than 0, it is converted to true.
The result of the conversion of name depends on the input value of the parameter.
Then the return value of || is assiged to name on the left hand side of =. This is because the precedence of || is greater than that of =.

This means that if you pass a value that can be converted to false, like 0 or '', Hello, <Your name> will be printed by sayHello() function.