leifw: Oddities of Boolean evaluation in JavaScript

Wednesday, August 19, 2009

Oddities of Boolean evaluation in JavaScript

I had some naive notions about how JavaScript evaluated certain Boolean expressions. For example, I assumed that code like

if (x) { ... }

would always evaluate like

if (x == true) { ... }

I was wrong. For example,

if ([]) { ... }

evaluates to true, but

if ([] == true) { ... }

evaluates to false.

I wrote the following test to help me identify other perilous JavaScript irregularities.
Read the squares as:

if (y)
    green
else
    red

Most Interesting

[]

[] == true

[] != undefined

[0]

[0] == true

[0] != undefined

[1]

[1] == true

[1] != undefined

0 == true

0 != undefined

1 == true

1 != undefined

'' == true

'' != undefined

'1'

'1' == true

'1' != undefined

'0'

'0' == true

'0' != undefined

false

false == true

false != undefined

true

true == true

true != undefined

'false'

'false' == true

'false' != undefined

'true'

'true' == true

'true' != undefined

null

null == true

null != undefined

undefined

undefined == true

undefined != undefined

Double equal false things

[] == undefined

[] == null

[] == false

[0] == undefined

[0] == null

[0] == false

[1] == undefined

[1] == null

[1] == false

0 == undefined

0 == null

0 == false

1 == undefined

1 == null

1 == false

'' == undefined

'' == null

'' == false

'1' == undefined

'1' == null

'1' == false

'0' == undefined

'0' == null

'0' == false

false == undefined

false == null

false == false

true == undefined

true == null

true == false

'false' == undefined

'false' == null

'false' == false

'true' == undefined

'true' == null

'true' == false

null == undefined

null == null

null == false

undefined == undefined

undefined == null

undefined == false

Not equal false things

[] != undefined

[] != null

[] != false

[0] != undefined

[0] != null

[0] != false

[1] != undefined

[1] != null

[1] != false

0 != undefined

0 != null

0 != false

1 != undefined

1 != null

1 != false

'' != undefined

'' != null

'' != false

'1' != undefined

'1' != null

'1' != false

'0' != undefined

'0' != null

'0' != false

false != undefined

false != null

false != false

true != undefined

true != null

true != false

'false' != undefined

'false' != null

'false' != false

'true' != undefined

'true' != null

'true' != false

null != undefined

null != null

null != false

undefined != undefined

undefined != null

undefined != false

Double equal true

[]

[] == true

[] != true

[0]

[0] == true

[0] != true

[1]

[1] == true

[1] != true

0 == true

0 != true

1 == true

1 != true

'' == true

'' != true

'1'

'1' == true

'1' != true

'0'

'0' == true

'0' != true

false

false == true

false != true

true

true == true

true != true

'false'

'false' == true

'false' != true

'true'

'true' == true

'true' != true

null

null == true

null != true

undefined

undefined == true

undefined != true

Triple equal true

[] === true

[] !== true

[0] === true

[0] !== true

[1] === true

[1] !== true

0 === true

0 !== true

1 === true

1 !== true

'' === true

'' !== true

'1' === true

'1' !== true

'0' === true

'0' !== true

false === true

false !== true

true === true

true !== true

'false' === true

'false' !== true

'true' === true

'true' !== true

null === true

null !== true

undefined === true

undefined !== true

Triple equal false things

[] === undefined

[] !== undefined

[] === null

[] !== null

[] === false

[] !== false

[0] === undefined

[0] !== undefined

[0] === null

[0] !== null

[0] === false

[0] !== false

[1] === undefined

[1] !== undefined

[1] === null

[1] !== null

[1] === false

[1] !== false

0 === undefined

0 !== undefined

0 === null

0 !== null

0 === false

0 !== false

1 === undefined

1 !== undefined

1 === null

1 !== null

1 === false

1 !== false

'' === undefined

'' !== undefined

'' === null

'' !== null

'' === false

'' !== false

'1' === undefined

'1' !== undefined

'1' === null

'1' !== null

'1' === false

'1' !== false

'0' === undefined

'0' !== undefined

'0' === null

'0' !== null

'0' === false

'0' !== false

false === undefined

false !== undefined

false === null

false !== null

false === false

false !== false

true === undefined

true !== undefined

true === null

true !== null

true === false

true !== false

'false' === undefined

'false' !== undefined

'false' === null

'false' !== null

'false' === false

'false' !== false

'true' === undefined

'true' !== undefined

'true' === null

'true' !== null

'true' === false

'true' !== false

null === undefined

null !== undefined

null === null

null !== null

null === false

null !== false

undefined === undefined

undefined !== undefined

undefined === null

undefined !== null

undefined === false

undefined !== false

Labels: javascript

posted by Leif Wickland @ 11:16 AM

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