var a='1,125'
a=a.replace(/\,/g,'')
a=Number(a)

Replace the regex in the replace method with /,/g which means matches the character,literally (case sensitive)

var num = '12,312,313,214,214,324.89';
var num2 = '12,312,313,214,214,324';

function replaceComma(num) {
  return num.replace(/,/g, '');
};
console.log(replaceComma(num));
console.log(replaceComma(num2));

Can't you do something like this then,

Altough the dollar sign is not included:

var num="1,000,00";
var getin=true;
var concatenate="";
for (var i=0; i<num.length; i++)
{
    if (num[i]=="," && getin){ getin=false;continue;}
    concatenate+=num[i];
}
alert(parseInt(concatenate));

It would also work for values greater then 9999 ect..

Related answer, but if you want to run clean up a user inputting values into a form, here's what you can do:

const numFormatter = new Intl.NumberFormat('en-US', {
  style: "decimal",
  maximumFractionDigits: 2
})

// Good Inputs
parseFloat(numFormatter.format('1234').replace(/,/g,"")) // 1234
parseFloat(numFormatter.format('123').replace(/,/g,"")) // 123

// 3rd decimal place rounds to nearest
parseFloat(numFormatter.format('1234.233').replace(/,/g,"")); // 1234.23
parseFloat(numFormatter.format('1234.239').replace(/,/g,"")); // 1234.24

// Bad Inputs
parseFloat(numFormatter.format('1234.233a').replace(/,/g,"")); // NaN
parseFloat(numFormatter.format('$1234.23').replace(/,/g,"")); // NaN

// Edge Cases
parseFloat(numFormatter.format(true).replace(/,/g,"")) // 1
parseFloat(numFormatter.format(false).replace(/,/g,"")) // 0
parseFloat(numFormatter.format(NaN).replace(/,/g,"")) // NaN

Use the international date local via format. This cleans up any bad inputs, if there is one it returns a string of NaN you can check for. There's no way currently of removing commas as part of the locale (as of 10/12/19), so you can use a regex command to remove commas using replace.

ParseFloat converts the this type definition from string to number

If you use React, this is what your calculate function could look like:

updateCalculationInput = (e) => {
    let value;
    value = numFormatter.format(e.target.value); // 123,456.78 - 3rd decimal rounds to nearest number as expected
    if(value === 'NaN') return; // locale returns string of NaN if fail
    value = value.replace(/,/g, ""); // remove commas
    value = parseFloat(value); // now parse to float should always be clean input

    // Do the actual math and setState calls here
}