Editing JavaScript (section)

=== Weakly typed ===
{{Main|Weakly typed}}

JavaScript is [[Strong and weak typing|weakly typed]], which means certain types are implicitly cast depending on the operation used.<ref name="casting_rules">{{cite web |last=Korolev |first=Mikhail |date=2019-03-01 |title=JavaScript quirks in one image from the Internet |url=https://dev.to/mkrl/javascript-quirks-in-one-image-from-the-internet-52m7 |url-status=live |archive-url=https://web.archive.org/web/20191028204723/https://dev.to/mkrl/javascript-quirks-in-one-image-from-the-internet-52m7 |archive-date=October 28, 2019 |access-date=October 28, 2019 |website=The DEV Community |language=en}}</ref>

* The binary <code>+</code> operator casts both operands to a string unless both operands are numbers. This is because the addition operator doubles as a concatenation operator
* The binary <code>-</code> operator always casts both operands to a number
* Both unary operators (<code>+</code>, <code>-</code>) always cast the operand to a number. However, <code>+</code> always casts to <code>Number</code> ([[Double-precision floating-point format|binary64]]) while <code>-</code> preserves <code>BigInt</code> ([[Arbitrary-precision arithmetic|integer]])<ref>{{cite web | url=https://github.com/tc39/proposal-bigint/blob/master/ADVANCED.md#dont-break-asmjs | title=Proposal-bigint/ADVANCED.md at master · tc39/Proposal-bigint | website=[[GitHub]] }}</ref>

Values are cast to strings like the following:<ref name="casting_rules" />

* Strings are left as-is
* Numbers are converted to their string representation
* Arrays have their elements cast to strings after which they are joined by commas (<code>,</code>)
* Other objects are converted to the string <code>[object Object]</code> where <code>Object</code> is the name of the constructor of the object

Values are cast to numbers by casting to strings and then casting the strings to numbers. These processes can be modified by defining <code>toString</code> and <code>valueOf</code> functions on the [[#Object-orientation (prototype-based)|prototype]] for string and number casting respectively.

JavaScript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency.<ref>{{cite web |date=2012 |title=Wat |url=https://www.destroyallsoftware.com/talks/wat |url-status=live |archive-url=https://web.archive.org/web/20191028204723/https://www.destroyallsoftware.com/talks/wat |archive-date=October 28, 2019 |access-date=October 28, 2019 |website=Destroy All Software |first1=Gary |last1=Bernhardt  }}</ref><ref name="casting_rules" /> For example, when adding a number to a string, the number will be cast to a string before performing concatenation, but when subtracting a number from a string, the string is cast to a number before performing subtraction.

{| class="wikitable"
|+JavaScript type conversions
!left operand
!operator
!right operand
!result
|-
|<code>[]</code> (empty array)
|<code>+</code>
|<code>[]</code> (empty array)
|<code>""</code> (empty string)
|-
|<code>[]</code> (empty array)
|<code>+</code>
|<code>{}</code> (empty object)
|<code>"[object Object]"</code> (string)
|-
|<code>false</code> (boolean)
|<code>+</code>
|<code>[]</code> (empty array)
|<code>"false"</code> (string)
|-
|<code>"123"</code>(string)
|<code>+</code>
|<code>1</code> (number)
|<code>"1231"</code> (string)
|-
|<code>"123"</code> (string)
|<code>-</code>
|<code>1</code> (number)
|<code>122</code> (number)
|-
|<code>"123"</code> (string)
|<code>-</code>
|<code>"abc"</code> (string)
|<code>[[NaN]]</code> (number)
|}

Often also mentioned is <code>{} + []</code> resulting in <code>0</code> (number). This is misleading: the <code>{}</code> is interpreted as an empty code block instead of an empty object, and the empty array is cast to a number by the remaining unary <code>+</code> operator. If the expression is wrapped in parentheses - <code>({} + [])</code> – the curly brackets are interpreted as an empty object and the result of the expression is <code>"[object Object]"</code> as expected.<ref name="casting_rules" />