** A function**
in JavaScript is similar to a procedure—a set of statements that performs a task or calculates a value, but for a procedure to qualify as a function, it should take some input and return an output where there is some obvious relationship between the input and the output. To use a function, you must define it somewhere in the scope from which you wish to call it.
JavaScript Function Syntax A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().
Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).
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The code to be executed, by the function, is placed inside curly brackets: {}
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Function parameters are listed inside the parentheses () in the function definition.
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Function arguments are the values received by the function when it is invoked.
** Function Invocation **
The code inside the function will execute when "something" invokes (calls) the function:
Function Return
When JavaScript reaches a return statement, the function will stop executing.
If the function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.
Functions often compute a return value. The return value is "returned" back to the "caller":
Why Functions?
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You can reuse code: Define the code once, and use it many times.
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You can use the same code many times with different arguments, to produce different results.