JavaScript specials

This chapter briefly recaps the features of JavaScript that we’ve learned by now, paying special attention to subtle moments.

Code structure

Statements are delimited with a semicolon:

alert('Hello'); alert('World');

Usually, a line-break is also treated as a delimiter, so that would also work:

alert('Hello')
alert('World')

That’s called “automatic semicolon insertion”. Sometimes it doesn’t work, for instance:

alert("There will be an error after this message")

[1, 2].forEach(alert)

Most codestyle guides agree that we should put a semicolon after each statement.

Semicolons are not required after code blocks {...} and syntax constructs with them like loops:

          function f() {
  // no semicolon needed after function declaration
}

for(;;) {
  // no semicolon needed after the loop
}
        

…But even if we can put an “extra” semicolon somewhere, that’s not an error. It will be ignored.

Strict mode

To fully enable all features of modern JavaScript, we should start scripts with "use strict".

          'use strict';

...

The directive must be at the top of a script or at the beginning of a function body.

Without "use strict", everything still works, but some features behave in the old-fashioned, “compatible” way. We’d generally prefer the modern behavior.

Some modern features of the language (like classes that we’ll study in the future) enable strict mode implicitly.

Variables

Can be declared using:

let
const (constant, can’t be changed)
var (old-style, will see later)

A variable name can include:

Letters and digits, but the first character may not be a digit.
Characters $ and _ are normal, on par with letters.
Non-Latin alphabets and hieroglyphs are also allowed, but commonly not used.

Variables are dynamically typed. They can store any value: