Question

Say I have a pure constructor function (containing nothing but this.Bar = bar)

1) When I call it from another function, can I pass the caller function’s arguments directly when I call or must I do var myBar=new bar, myBar.Bar=thebar, where the bar is a caller argument?

2) Will the constructor still instantiate even if it doesn’t get all the args?

3) How can I check if one of the args is unique, IE no other instance of the object has this value for the property in question? Specifically, I want to assign each object a unique index at creation. Maybe array?

Many thanks in advance

Answer 1

Say I have a pure constructor function (containing nothing but this.Bar = bar)

I’m going to assume you mean:

function MyConstructor(bar) {
    this.Bar = bar;
}

(Note: The overwhelming convention in JavaScript is that property names start with a lower-case letter. So this.bar, not this.Bar. Initially-capped identifiers are usually reserved for constructor functions.)

1) When I call it from another function, can I pass the caller function’s arguments directly when I call or must I do var myBar=new bar, myBar.Bar=thebar, where the bar is a caller argument?

You can pass them directly:

function foo(a, b, c) {
    var obj = new MyConstructor(b);
}

2) Will the constructor still instantiate even if it doesn’t get all the args?

The number of arguments passed is not checked by the JavaScript engine. Any formal arguments that you don’t pass will have the value undefined when the function is called:

function MyConstructor(bar) {
    console.log(bar);
}
var obj = new MyConstructor(); // logs "undefined"

3) How can I check if one of the args is unique, IE no other instance of the object has this value for the property in question? Specifically, I want to assign each object a unique index at creation. Maybe array?

In general, that’s usually not in-scope for a constructor. But yes, you could use an array, or an object, to do that.

var knownBars = [];
function MyConstructor(bar) {
    if (knownBars.indexOf(bar) !== -1) {
        // This bar is known
    }
    else {
        // Remember this bar
        knownBars.push(bar);
    }
}

Of course, indexOf may not be what you want for searching, so you may need to use some other method of Array.prototype or your own loop.

Another way would be to use an object; this assumes that bar is a string or something that can usefully be turned into a string:

var knownBars = {};
function MyConstructor(bar) {
    if (knownBars.indexOf(bar) !== -1) {
        // This bar is known
    }
    else {
        // Remember this bar
        knownBars[bar] = 1;
    }
}