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Javascript in Ten Minutes

Spencer Tipping

December 8, 2011

Contents

1 Introduction 3

2 Types 3

3 Functions 4

3.1 Variadic behavior (a cool thing) . . . . . . . . . . . . . . . . . . . 4

3.2 Lazy scoping (a cool thing) . . . . . . . . . . . . . . . . . . . . . . 4

3.3 The meaning of this (the egregious disaster) . . . . . . . . . . . 5

3.3.1 Important consequence: eta-reduction . . . . . . . . . . . 6

3.3.2 Odd tidbit: this is never falsy . . . . . . . . . . . . . . . 7

4 Gotchas 7

4.1 Semicolon inference . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.2 Void functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.3 var . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.4 Lazy scoping and mutability . . . . . . . . . . . . . . . . . . . . . 8

4.5 Equality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.6 Boxed vs. unboxed . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.7 Things that will silently fail or misbehave . . . . . . . . . . . . . 10

4.8 Numeric coercion . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.9 Things that will loudly fail . . . . . . . . . . . . . . . . . . . . . . 13

4.10 Throwing things . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.11 Be careful with typeof . . . . . . . . . . . . . . . . . . . . . . . . 14

4.12 Also be careful with instanceof . . . . . . . . . . . . . . . . . . 15

4.13 Browser incompatibilities . . . . . . . . . . . . . . . . . . . . . . 15

5 Prototypes 16

5.1 Why new is awful . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.2 Why new isn’t quite so awful . . . . . . . . . . . . . . . . . . . . . 17

5.3 Why you should use prototypes . . . . . . . . . . . . . . . . . . . 18

5.4 Autoboxing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1

1 Introduction

This guide is for anyone who knows some Javascript but would like a quick

1

intro to its advanced features. It will be easier reading if you also know another

functional language such as Ruby, Perl, Python, ML, Scheme, etc, since I don’t

really explain how ﬁrst-class functions work.

2 Types

Javascript has nine types. They are:

1. Null – null. Chucks a wobbly if you ask it for any attributes; e.g. null.foo

fails. Never boxed.

2

2. Undeﬁned – undefined. What you get if you ask an object for something

it doesn’t have; e.g. document.nonexistent. Also chucks a wobbly if you

ask it for any attributes. Never boxed.

3. Strings – e.g. ’foo’, "foo" (single vs. double quotation marks makes no

diﬀerence). Sometimes boxed. Instance of String when boxed.

4. Numbers – e.g. 5, 3e+10 (all numbers behave as ﬂoats – signiﬁcant for

division, but can be truncated by x >>> 0). Sometimes boxed. Instance

of Number when boxed.

5. Booleans – true and false. Sometimes boxed. Instance of Boolean when

boxed.

6. Arrays – e.g. [1, 2, "foo", [3, 4]]. Always boxed. Instance of Array.

7. Objects – e.g. {foo: ’bar’, bif: [1, 2]}, which are really just hashta-

bles. Always boxed. Instance of Object.

8. Regular expressions – e.g. /foo\s*([bar]+)/. Always boxed. Instance

of RegExp.

9. Functions – e.g. function (x) {return x + 1}. Always boxed. In-

stance of Function.

The value null is actually almost never produced by Javascript. The only

case you’re likely to run across null is if you assign it somewhere (most of the

time you’ll get undefined instead – one notable exception is document.getElementById,

which returns null if it can’t ﬁnd an element). Making sparing use of undefined

and instead using null can make bugs much easier to track down.

1

Longer than ten minutes, despite what the title says.

2

Boxing is just a way of saying whether something has a pointer. A boxed type is a reference

type, and an unboxed type is a value type. In Javascript, this has additional ramiﬁcations as well –

see section 4.6.

3

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