MEPX
Chapter 2 of 16All chapters
  1. 1Introduction
  2. 2Syntax and statements
  3. 3Variables and scope
  4. 4Data types
  5. 5Comparison and coercion
  6. 6Strings
  7. 7Numbers and maths
  8. 8Arrays
  9. 9Objects
  10. 10Functions
  11. 11Conditions and loops
  12. 12Classes
  13. 13The DOM
  14. 14Events
  15. 15Promises and async
  16. 16Modules

Chapter 2 of 16

Syntax and statements

Semicolons, blocks and strict mode.

Shape of a program

Statements end with a semicolon, which the engine will insert for you if you leave it out. That mostly works, and the exceptions are annoying enough that most teams either always write them or run a formatter.

  • Braces group statements into a block, and blocks matter for let and const.
  • Comments are // to end of line and /* ... */ across lines.

Strict mode and modules

Modules and class bodies run in strict mode automatically, which turns silent mistakes into errors. In an old script you can opt in by putting "use strict" at the top.