How to create a function that will calculate a string? [duplicate]

I'm in the midst of creating a function operate(string) that will accept an input like operate("1 + 4 - 2') or operate("1 * 6 / 3) that will return the mathematically calculated value, in these examples it would be 4 or 1. Here is my JavaScript code so far:

// This functionfunction operate(string) {    let numbers = string.split(" ")    let operators = []    numbers.map((item, index) => {        if(item.match(/^[0-9]+$/) != null) {            numbers[index] = Number(item)        }    })    console.log(numbers)                        // [ 141, '+', 12, '-', 2 ]    let total = numbers[0]                      // 141    numbers.splice(0, 1)    let shortOperation;    // [ '+', 12 ]    for(let i = 0; i <= 2; i++) {        shortOperation = numbers.slice(0, 2)        numbers.splice(0, 3)                    // [ '-', 2 ]        switch(shortOperation[0]) {            case "+":                total += shortOperation[1]            case "-":                total +- shortOperation[1]            case "*":                total *= shortOperation[1]            case "/":                total /= shortOperation[1]            }    }    return total}const sample = "141 + 12 - 2"console.log(operate(sample))

For some apparent reason, it returns 153, when it should be 151. Why, and how can this issue be resolved (and don't worry about the order of operations, it's fine as it is with a left-to-right approach)?

I've been at it for about 2 hours, with minimal success.