JavaScript syntax: Difference between revisions

Revision as of 01:40, 1 October 2013

Revision as of 08:40, 9 November 2013

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===Whitespace and semicolons===

[[Space (punctuation)|Space]]s, [[tab character|tab]]s and [[newline]]s used outside of string constants are called [[whitespace (computer science)|whitespace]]. Unlike C, whitespace in JavaScript source can directly impact semantics. Because of a technique called "automatic semicolon insertion" (ASI), some statements that are well formed when a newline is parsed will be considered complete (as if a semicolon were inserted just prior to the newline). ProgrammersSome areauthorities advisedadvise to supplysupplying statement-terminating semicolons explicitly because it may lessen unintended effects of the automatic semicolon insertion.<ref>{{cite book

|title=JavaScript: The definitive Guide

|last=Flanagan

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// Treated as:

// a = b + c(d + e).foo();

</syntaxhighlight>

Alternatively, some suggest the use of ''leading'' semicolons on lines starting with ( or <nowiki>[</nowiki>, so the line is not accidentally joined with the previous one. This is known as a '''defensive semicolon''', and is particularly recommended because code may become ambiguous when it is rearranged. For example:

<syntaxhighlight lang="JavaScript">

a = b + c

;(d + e).foo()

// Treated as:

// a = b + c;

// (d + e).foo();

</syntaxhighlight>