Javascript Function Problems

Javascript - Function Problems

Javascript is a highly non-standard language. Basic function calls demonstrate this.

Default Parameter Values | By Value vs By Reference | Function Definitions

Default Parameter Values

Most languages provide a simple technique to provide default values for function parameters. The following technique works fine in Chrome and Firefox, but causes a hard error in Microsoft IE and Microsoft Edge.

TmcUCAR.prototype.PlotData = function(label, x, y, color, n, vertAxis="left") {

The "error" is

Message: Expected ')'
Line: 382
Char: 70
Code: 0
URI: http://www.xyz.com/[path]/abc.js

Column 70 is the equals sign (=).

Using an equals sign (=) is part of ECMAScript 6 which was mostly supported by browsers after 2018. (This page was started 01-2017.)

w3schools.com suggests the following syntax.

function myFunction(x, y) {
  if (y === undefined) {
    y = 0;
  }
}

However, this should also work (observed on github).

function myFunction(x, y) {
  y = y || 0;
}

By Value vs By Reference

According to w3schools.com,

Function arguments are passed by value, but
Objects (including arrays) are passed by reference

Basically,

Changes made to arguments passed by value are not available outside the current instance of the function
Changes made to arguments passed by reference remain available outside the current function instance

This is a real pain, all the better languages allow the programmer to control whether arguments are passed "by value" or "by reference".

Function Definitions

According to w3schools.com, when functions are defined, a semicolon is NOT required after the final curly brace. However, when a variable is assigned to an anonymous function (a function without a name), that is actually an executable statement and a semicolon IS required.

function myFunction(a, b) { return a * b; } // named function, no semicolon

var x = function (a, b) {return a * b};     // a variable is assigned to an anonymous function
                                            // and a semicolon IS required

However, in all the browsers I've tried, the final semicolon is NOT required in either case.

When "use strict"; is specified, then the var is required for both regular variables and function variables. However, when adding methods (subroutines) to objects, it is not

"use strict";

function TmcSomeObject(){ // a function is an object, it can be expanded using ".prototype."
 // this could be null
}

TmcSomeObject.prototype.init = function( ) { // this adds a method to an existing object
  // the real code goes here
}

count = 5;           // this fails - Uncaught ReferenceError: count is not defined
func1 = function(){} // this fails - Uncaught ReferenceError: func1 is not defined

var func1 = function(){} // this works

Author: Robert Clemenzi