Format number to always show 2 decimal places

(Math.round(num * 100) / 100).toFixed(2);

Live Demo

var num1 = "1";
document.getElementById('num1').innerHTML = (Math.round(num1 * 100) / 100).toFixed(2);

var num2 = "1.341";
document.getElementById('num2').innerHTML = (Math.round(num2 * 100) / 100).toFixed(2);

var num3 = "1.345";
document.getElementById('num3').innerHTML = (Math.round(num3 * 100) / 100).toFixed(2);

span {
    border: 1px solid #000;
    margin: 5px;
    padding: 5px;
}

<span id="num1"></span>
<span id="num2"></span>
<span id="num3"></span>

Note that it will round to 2 decimal places, so the input 1.346 will return 1.35.

For modern browsers, use toLocaleString:

var num = 1.345;
num.toLocaleString(undefined, { maximumFractionDigits: 2, minimumFractionDigits: 2 });

Specify a locale tag as first parameter to control the decimal separator. For a dot, use for example English U.S. locale:

num.toLocaleString("en-US", { maximumFractionDigits: 2, minimumFractionDigits: 2 });

which gives:

1.35

Most countries in Europe use a comma as decimal separator, so if you for example use Swedish/Sweden locale:

num.toLocaleString("sv-SE", { maximumFractionDigits: 2, minimumFractionDigits: 2 });

it will give:

1,35

Number(1).toFixed(2);         // 1.00
Number(1.341).toFixed(2);     // 1.34
Number(1.345).toFixed(2);     // 1.34 NOTE: See andy's comment below.
Number(1.3450001).toFixed(2); // 1.35

document.getElementById('line1').innerHTML = Number(1).toFixed(2);
document.getElementById('line2').innerHTML = Number(1.341).toFixed(2);
document.getElementById('line3').innerHTML = Number(1.345).toFixed(2);
document.getElementById('line4').innerHTML = Number(1.3450001).toFixed(2);

<span id="line1"></span>
<br/>
<span id="line2"></span>
<br/>
<span id="line3"></span>
<br/>
<span id="line4"></span>

This answer will fail if value = 1.005.

As a better solution, the rounding problem can be avoided by using numbers represented in exponential notation:

Number(Math.round(1.005+'e2')+'e-2'); // 1.01

Cleaner code as suggested by @Kon, and the original author:

Number(Math.round(parseFloat(value + 'e' + decimalPlaces)) + 'e-' + decimalPlaces)

You may add toFixed() at the end to retain the decimal point e.g: 1.00 but note that it will return as string.

Number(Math.round(parseFloat(value + 'e' + decimalPlaces)) + 'e-' + decimalPlaces).toFixed(decimalPlaces)

Credit: Rounding Decimals in JavaScript