In this section, we show only the very basics of JavaScript. For more details, see here.
Items on a (light) grey background are for more advanced use and not (so) relevant for the Challenge.
In the column How, we show how to express something in JavaScript. Text in italics needs to be replaced by appropriate code fragments.
| Lexical Elements (See JavaScript coding conventions for good advice on how to format a program text.) | ||
|---|---|---|
| What | How | Remarks |
| Comments |
/* comment_text */ // comment text until line end | Comment text between /* */ can span multiple lines. Is used to explain code fragments, or to disable code fragments temporarily |
| Numeric Literals |
0 1 3.14 -1.41421 | |
| String Literals |
'characters-without-single-quote' | Can contain only printable characters, and no single quotes. N.B. String values can contain any character. |
| Identifiers (Names) |
n pi input a4 | Used to refer to variables and functions. Do not start with a digit. Cannot be a keyword. Are case sensitive. |
| Keywords |
var if else while function return | N.B. There are some more keywords, but we do not treat them here. |
| Operators |
+ - * / == != = | See Expressions and
Statements for their use and meaning. |
| Punctuators |
( ) , { } ;
| |
| Expressions | ||
| What | How | Remarks |
| Grouping (Nesting) |
( expr ) | To force grouping, overruling the default grouping |
| Value of a name |
name | Of a variable |
| Equality test |
expr == expr | Boolean expression, valid or not N.B. Do not confuse with assignment = |
| Inquality test |
expr != expr | Boolean expression, valid or not |
| Arithmetic operations on numbers |
num_expr + num_expr num_expr - num_expr - num_expr num_expr * num_expr num_expr / num_expr | Addition Subtraction Negation Multiplication Division |
| String operations |
str_expr + str_expr str_expr.length str_expr.charAt(num_expr) str_expr.substr(num_expr, num_expr) str_expr.substr(num_expr) | String concatenation String length Character indexing starts at 0 s.substr(i, n) returns n characters of s from index i; thus, s.charAt(i) == s.substr(i, 1) s.substr(i) returns all characters of s from index i on |
| Function call |
func_name(argument_list) | An argument list is a possibly empty comma-separated list of expressions. The values of these expressions are bound to the corresponding parameter names, and the statement list of the function is executed. The value is that of the first return expression encountered during execution. |
| Declarations and Definitions | ||
| What | How | Remarks |
| Variable declaration |
var name; var name = expression; | Without expression, the initial value is undefined. |
| Function definition |
function name(parameter_list) {
declarations_and_definitions
statement_list
}
| A parameter list is a possibly empty comma-separated list of distinct names. In a function call, these names obtain the arguments as initial value. Also see the return statement. |
| Statements | ||
| What | How | Remarks |
| Assignment |
var_name = expression; | Expression value becomes new variable value. Note the trailing semicolon (;) |
| Conditional Execution |
if ( bool_expr ) {
statement_list
}
else if ( bool_expr ) {
statement_list
}
else {
statement_list
}
| The boolean expressions are evaluated in order until a valid one is encountered; the corresponding statement list is executed. The else and else-if parts are optional. There can be multiple else-if sections. |
| Conditional Repetition |
while ( bool_expr ) {
statement_list
}
| The boolean expression is first evaluated. The repetition terminates when it is not valid. If it is valid, then the statement list is executed, and the process is repeated. |
| Return from function |
return expression; return; | Without expression, the function value is undefined (useful when the function is to be called as statement). |
| Function call |
func_name(argument_list); | An argument list is a possibly empty comma-separated list of expressions. The values of these expressions are bound to the corresponding parameter names, and the statement list of the function is executed. Execution terminates at the end of the statement list, or at the first return encountered; the return value is ignored. |
| Interactive Input and Output (useful for locating errors) | ||
| What | How | Remarks |
| Output window |
alert(s); | Shows string s in a popup window |
| Confirmation window |
confirm(s); | Shows string s in a popup window, and lets the user Cancel or OK, returning the boolean choice as function value. Used in conjunction with if or while statement. |
| Input window |
prompt(s); | Shows string s in a popup window, and lets the user type in a string, that is returned as function value |
| Extensions for
Tom's JavaScript Machine (See Operating Instructions for details.) | ||
| What | How | Remarks |
| Process input |
input readStr() readNum() moreInputs() separator inputs nextInputIndex | |
| Produce output |
write()
writeln()
writequ()
output
| |
| Functions |
isQuote(c) isNewline(c) | |
| Constants |
QU NL | |