Why can functions be called without parentheses when using template strings?

Question

I have a simple logging function:

function log(str) {
  console.log('logged: ', str);
}

If I call it without parentheses (currently using Chrome's dev tools) and pass in a template string, like this:

log`foo`

The output is: logged: ["foo", raw: Array[1]]

If I call it with parentheses,

log(`foo`)

The output is: logged: foo

Why does calling a function using a template string an no parentheses work in Javascript? What is happening that causes the result to be different from calling it with parentheses?

Answer 1

The first example (log`foo`) allows the language specification to determine the values passed to the log function (See 12.3.7). The second example (log(`foo`)) explicitly passes a single argument.

Template literals can contain strings, as well as expressions. Sometimes you may wish to have more control over the compilation of a string from its string-parts, and expression-parts. In this case, you may be looking for tagged templates.

var name = "Jonathan";
var mssg = foo `Hello, ${name}. Nice name, ${name}.`;

function foo ( strings, ...values ) {
    console.log( strings ); //["Hello, ", ". Nice name, ", ".", raw: Array[3]]
    console.log( values  ); //["Jonathan", "Jonathan"]
}

Notice here how all of the strings are passed in through the first argument. As well, all of the interpolated value expressions are passed in through the rest of the parameters (pulled together into an array here).

With this added control, we could do all sorts of things, such as localization. In this example, the language specification determines the appropriate values to pass to the function — the developer doesn't determine this.

To contrast, when you call log(foo), you wind up getting only the resulting string. No objects, no parts, no raw values.