JavaScript syntax: Difference between revisions

Revision as of 16:48, 26 January 2021

Revision as of 19:07, 29 January 2021

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When JavaScript tries to '''resolve''' an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the ''scope chain'' until it reaches the ''global scope'' where global variables reside. If it is still not found, JavaScript will raise a <code>ReferenceError</code> exception.

When '''assigning''' an identifier, JavaScript goes through exactly the same process to retrieve this identifier, except that if it is not found in the ''global scope'', it will create the "variable" in the scope where it was created. <ref>ECMA-262 5e edition clarified this behavior with the ''Declarative Environment Record'' and ''Object Environment Record''. With this formalism, the ''global object'' is the ''Object Environment Record'' of the global ''Lexical Environment'' (the ''global scope'').</ref> As a consequence, a variable never declared will be global, if assigned. Declaring a variable (with the keyword <code>var</code>) in the ''global scope'' (i.e. outside of any function body (or block in the case of let/const)), assigning a never declared identifier or adding a property to the ''global object'' (usually ''window'') will also create a new global variable.

Note that JavaScript's ''strict mode'' forbids the assignment of an undeclared variable, which avoids global namespace pollution. <!-- Removed, unrelated to the first statement and sentence was already stated. Delete this comment if you edit this, this is just an explanation for my big edit. -->

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Here, calling <code>isUndefined(my_var)</code> raises a {{mono|ReferenceError}} if {{mono|my_var}} is an unknown identifier, whereas {{code|lang=javascript|code=typeof my_var === 'undefined'}} doesn't.

===Number===

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!Property!!Returned value<br />rounded to 5 digits!!Description

|-

| <tt>Math.E</tt> || align=center|2.7183 || {{mvar|[[e (mathematical constant)|{{mvar|e}}]]}}: Natural logarithm base

|-

| <tt>Math.LN2</tt> || align=center|0.69315 || [[Natural logarithm]] of 2

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str2 = "2" + 2; // "22", not "4" or 4.

</syntaxhighlight>

===??===

{{excerpt|Null coalescing operator|JavaScript}}