Feb 16, 2017 3:00:55 AM | JavaScript Error Handling - SyntaxError: "x" is not a legal ECMA-262 octal constant

A detailed exploration of octals, numeric literals, and the x is not a legal ECMA-262 octal constant SyntaxError in JavaScript.

Continuing right along through our JavaScript Error Handling series, today we'll be taking a closer look at the Invalid Octal Constant warning. The Invalid Octal Constant warning is quite unique in the realm of all the JavaScript errors we've explored thus far, because it only appears in two very specific instances: when defining an octal literal value of 08 or 09.

Below we'll explore just what octal literals are, how an Invalid Octal Constant warning might then appear, and what to do to avoid these errors yourself. Let's get going!

The Technical Rundown

  • All JavaScript error objects are descendants of the Error object, or an inherited object therein.
  • The SyntaxError object is inherited from the Error object.
  • The Invalid Octal Constant warning is a specific type of SyntaxError object.

When Should You Use It?

Since the appearance of an Invalid Octal Constant warning deals with octal literals, we should first take a closer look at what octal literals are and why they're used. Time for a tiny bit of math (apologies all around)!

In short, the octal numeral system is a base-8 number system. This differs from the decimal system that we all use daily, which is base-10, meaning that every place represented by a decimal number is a power of ten. On the other hand, the octal system is base-8, so every place represents a power of eight.

To see this in action, first let's pick any decimal number, in this case we'll go with 99. Now, to convert from decimal to octal, we must take the binary representation of our decimal number (99), and group it into groups of three binary digits, beginning at the right side.

So, to begin, we start with the 99:

99 (decimal)
01100011 (binary)

Since we need groups of three binary digits to convert to octal, and our binary representation only contains eight digits in this case, we must add extra zeroes to the left side until we have a quantity of binary digits divisible by three. So here, we're adding one zero to the left side:

99 (decimal)
01100011 (binary)
001 100 011 (binary triplets)

Next, we convert each of those binary triplets into their octal equivalent. The trick here is that since the maximum value that a trio of binary digits can represent is 7 (111 in binary), we are now using octal representations within binary. So in this case, our binary triplets are converted to: 1 4 3

99 (decimal)
01100011 (binary)
001 100 011 (binary triplets)
1 4 3 (octal numerals)

If we squish it together, we get the octal numeral of 143, which is the octal equivalent to 99 in decimal! To prove this, we can simply take our separated octal numerals (1 4 3) and go through each one at a time, multiplying the numeral by the octal power represented by that place in the number. So the right-most place would multiplied by 8 raised to the power of 0, the next place would be multiplied by 8 raised to the power of 1, and so on down the line. The resulting calculation looks like this:

1 4 3 (octal numerals)

143 (octal) = 3 * 8^0 + 4 * 8^1 + 1 * 8^2
3 + 32 + 64

Whew! Now that we know what an octal is, how does this apply to JavaScript at the appearance of the Invalid Octal Constant warning? JavaScript allows for the expression of numeric literals to be defined in a variety of ways:

// With no lead
123; // decimal, base 10

// With leading zero, up to 7
04; // octal, base 8

// With leading zero, above 9
056; // decimal, base 10

// With leading 0x
0x1234 // hexadecimal, base 16

// With leading 0b
0b10 // binary, base 2

The problem, and where we can see an Invalid Octal Constant warning appear, is when attempting to create a literal with a leading zero that falls within that gap between octal and decimal: 08 or 09. JavaScript initially tries to convert a leading-zero literal to octal, but since we know that 08 and 09 are too large to be represented in octal (which is maximum of zero through seven), but too small to be decimal (which starts at ten), JavaScript throws the Invalid Octal Constant warning. Here's a simple example illustrating this:

var printError = function(error, explicit) {
console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);

try {
} catch (e) {
if (e instanceof SyntaxError) {
printError(e, true);
} else {
printError(e, false);

What's important to note here is that this isn't technically an error, but instead it's a warning. Therefore, in spite of our best efforts, we cannot catch this Invalid Octal Constant warning, and the engine spits out both a decimal number to the log, as well as the warning message:

SyntaxError: 08 is not a legal ECMA-262 octal constant

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Written By: Frances Banks