JavaScript Tutorials #07 part-2 (image Slide show using javascript)
javascript #07 part -2
Origins[edit]
Brendan Eich summarized the ancestry of the syntax in the first paragraph of the JavaScript 1.1 specification[1] as follows:
Basics[edit]
Case sensitivity[edit]
JavaScript is case sensitive. It is common to start the name of a constructor with a capitalised letter, and the name of a function or variable with a lower-case letter.
Whitespace and semicolons[edit]
Spaces, tabs and newlines used outside of string constants are called 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). Some authorities advise supplying statement-terminating semicolons explicitly, because it may lessen unintended effects of the automatic semicolon insertion.[2]
There are two issues: five tokens can either begin a statement or be the extension of a complete statement; and five restricted productions, where line breaks are not allowed in certain positions, potentially yielding incorrect parsing.[3]
The five problematic tokens are the open parenthesis "(", open bracket "[", slash "/", plus "+", and minus "-". Of these, open parenthesis is common in the immediately-invoked function expression pattern, and open bracket occurs sometimes, while others are quite rare. The example given in the spec is:[3]
a = b + c
(d + e).foo()
// Treated as:
// a = b + c(d + e).foo();
with the suggestion that the preceding statement be terminated with a semicolon.
Some suggest instead the use of leading semicolons on lines starting with '(' or '[', 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 otherwise become ambiguous when it is rearranged.[3][4] For example:
a = b + c
;(d + e).foo()
// Treated as:
// a = b + c;
// (d + e).foo();
Initial semicolons are also sometimes used at the start of JavaScript libraries, in case they are appended to another library that omits a trailing semicolon, as this can result in ambiguity of the initial statement.
The five restricted productions are return, throw, break, continue, and post-increment/decrement. In all cases, inserting semicolons does not fix the problem, but makes the parsed syntax clear, making the error easier to detect. return and throw take an optional value, while break and continue take an optional label. In all cases, the advice is to keep the value or label on the same line as the statement. This most often shows up in the return statement, where one might return a large object literal, which might be accidentally placed starting on a new line. For post-increment/decrement, there is potential ambiguity with pre-increment/decrement, and again it is recommended to simply keep these on the same line.
return
a + b;
// Returns undefined. Treated as:
// return;
// a + b;
// Should be written as:
// return a + b;
0 Comment:
Post a Comment