---
title: You Don’t Know JS - Ch.8-III
tags: Templates, Talk
description: View the slide with "Slide Mode".
---

# You Don’t Know JS - Ch.8-III

## Automatic Semicolons

ASI (Automatic Semicolon Insertion) allows JS to be tolerant of certain places where ; isn’t commonly thought to be necessary.

var a = 42, b = "foo"
c = a
a 
b 
c //42

---
Statement blocks do not require ; , so ASI isn’t necessary:

var a = 42;
do {
    a--;
    console.log(a);
} while (a>=0) // <-- no ; expected here!
a;

---
ASI kicks in is with the break, continue, return, and (ES6) yield keywords

For a example:
The return statement doesn’t carry across the newline to the a *= 2 expression, as ASI assumes the ; terminating the return statement.

function foo(a) {
    if (a) return
    a *= 2;
}
a // undefined

Compare with no API

function foo(a) {
    if (a) 
        return a *= 2;
}
a // a*2

---
## Error Correction

### Error
---
One simple example is with syntax inside a regular expression literal,but the invalid
regex will throw an early error:

var a = /+foo/; // Error!

---
The target of an assignment must be an identifier so
a value like 42 in that position is illegal and can be reported right away:

var a;
42 = a; // Error!

---
ES5’s strict mode defines even more early errors. For example, in strict mode, function parameter names cannot be duplicated:

function bar(a,b,a) { "use strict"; } // Error!

---
### Using Variables Too Early
---
#### TDZ (“Temporal Dead Zone”).

The TDZ refers to places in code where a variable reference cannot yet be made, because it hasn’t reached its required initialization.

{
    a = 2; // ReferenceError!
    let a;
}

While typeof has an exception to be safe for undeclared variables, no such safety exception is made for TDZ references:

{
    typeof a; // undefined
    typeof b; // ReferenceError! (TDZ)
    let b;
}

## Function Arguments
---
When using ES6’s default parameter values,the default value is applied to the parameter if you either omit an argument, or you pass an undefined value in its place.

function foo( a = 42, b = a + 1 ) {
console.log( a, b );
}
foo(); // 42 43
foo( undefined ); // 42 43
foo( 5 ); // 5 6
foo( void 0, 7 ); // 42 7
foo( null ); // null 1

---
If you pass an argument, the arguments slot and the named parameter are linked to always have the same value. If you omit the argument, no such linkage occurs.

function foo(a) {
a = 42;
console.log( arguments[0] );
}
foo( 2 ); // 42 (linked)
foo(); // undefined (not linked)

---
But in strict mode, the linkage doesn’t exist regardless:

function foo(a) {
"use strict";
a = 42;
console.log( arguments[0] );
}
foo( 2 ); // 2 (not linked)
foo(); // undefined (not linked)