Prototype method to check if a string is balanced in terms of opening and closing parentheses.

BalancedString.coffee

String.prototype.isBalanced = ->
    parens = this.match( /[()]/g )
    return false unless parens?
    openStarted = false
    closureBalance = 0
    for i in [0...parens.length] by 1
        char = parens[i]
        if openStarted
            if char == "(" then closureBalance++ else closureBalance--
        else
            if char == "("
                openStarted = true 
                closureBalance++
        return false if closureBalance < 0
    return closureBalance is 0
    
examples = [
    {
        string : '(x(x)'
        expectation : false     
    },
    {
        string : ')(x(x)'
        expectation : false     
    },
    {
        string : ')(x(x)x)'
        expectation : true    
    },
    {
        string : '(x)(x))x()'
        expectation : false     
    },
    {
        string : '((x)(x))x()'
        expectation : true     
    },
]

for i in [0...examples.length] by 1
    ex = examples[i]
    console.log ex.string.isBalanced() == ex.expectation