Semicolon

• A semicolon may be omitted in most cases when a line break exists.
• There are situations where JavaScript “fails” to assume a semicolon where it is really needed.
• If we run the following code, only the first alert is shown and then we have an error.
  • The error in the no-semicolon variant occurs because JavaScript does not assume a semicolon before square brackets.

alert("There will be an error")
[1, 2].forEach(alert)

// Here's how the engine sees it:
alert("There will be an error")[1, 2].forEach(alert)

• It is recommended to put semicolons between statements even if they are separated by newlines.

The Modern Mode

• In 2009, ECMAScript 5 (ES5) added new features to the language and modified some of the existing ones.
• To keep the old code working, most modifications are off by default.
• You need to explicitly enable them with a special directive: "use strict"
  • When it is located at the top of a script, the whole script works the “modern” way.
• It can be put at the start of most kinds of functions instead of the whole script.
  • Doing that enables strict mode in that function only.
• There is no directive that reverts the engine to old behavior.
• Several language features, like “classes” and “modules”, enable strict mode automatically.