83 lines
2.4 KiB
Markdown
83 lines
2.4 KiB
Markdown
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# Chapter 2. Lexical Structure
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Cover:
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* Case sensitivity, space, and line breaks
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* Comments
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* Literals
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* Identifiers and reserved words
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* Unicode
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* Optional semicolons
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## 2.1 The Text of a JS Program
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* JS is **case-sensitive**.
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* JS ignore space character (`/u0020`)
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* JS ignore line breaks
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* JS recognizes tabs, assorted ASCII control chars, and Unicode space characters as whitespaces
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* JS recognizes newlines, carriage returns, and a carriage return/line feed sequences as line terminators.
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## 2.2 Comments
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2 types of comments supported:
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* `// comment content` for single-line comment
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* `/* comment content */` for multi-line comments
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## 2.3 Literals
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A **literal** (文字) is a data value that appears directly in a program. e.g.:
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* `12`
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* `1.2`
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* `'hello'`
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* `true`
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## 2.4 Identifiers and Reserved Words
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* **Identifiers (标识符)** is a just name to name constants, variables, functions, etc.
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* JS's identifiers start with
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* `_` or
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* `#` or
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* letter
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### 2.4.1 Reserved Words
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JS reserved some keywords, check [Mozilla reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Reserved_keywords_as_of_ECMAScript_2015)
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## 2.5 Unicode
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JS programs (files) are written using Unicode character set, so any Unicode charater can be used in string/comments.
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By convention, only ASCII letters/digits are used in identifiers
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### 2.5.1 Unicode Escape Sequence
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Some computer cannot use full set of Unicode char. For backward compatibility, JS defines **escape sequence** to write Unicode chars using only ASCII char.
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How? begin char with `\u` and followed by four hex digits (before ES6) or `{1 to 6 hex digit}` (since ES6):
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* `let cafe=1`
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* `caf\u00e9`
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* `caf\u{E9}`
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### 2.5.2 Unicode Normalization
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non-ASCII char in Unicode can be encoded into different form (introduced before), which create problem due to conflict. Hence Unicode normalization is required, using some tools or editor
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### 2.6 Optional Semicolons
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* JS use `;` to separate statements:
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* `a=3 ; b=4`
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* JS can omit semicolon if two statments are written on separate lines.
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* You can insist to write `;` for clear separation
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* JS treats a line break as a semicolon if the next nonspace char cannot be interpreted as a continuation of the current statement.
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Miss treatment of newline:
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```js
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let y = x + f
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(a+b).toString()
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```
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will be treated by JS as
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```js
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let y = x + f(a+b).toString();
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```
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