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function postFixEvaluation(expression) {

if (typeof expression !== 'string' || expression.length === 0) {

throw 'Invalid post fix expression.';

}

var stack = [];

for (var i = 0; i < expression.length; i++) {

var curr = expression[i];

if (curr.match(/^\d$/)) {

stack.push(curr - '0');

} else if (curr.match(/^[+-/*]$/)) {

if (stack.length < 2) {

throw 'Invalid post fix expression.';

}

var op = {

'+' : function(a, b) { return a + b },

'-': function(a, b) { return b - a },

'/': function(a, b) { return b / a },

'*': function(a, b) { return a * b }

};

var first = stack.pop();

var second = stack.pop();

stack.push(op[curr](first, second));

} else {

throw 'Invalid post fix expression.';

}

}

return stack.pop();

}

console.log(postFixEvaluation("235*+8-")); // === 9

console.log(postFixEvaluation("23/")); // === 0.66