Obtaining a value in a non-function JavaScript code block is achievable by

I'm currently studying the Box Plot example in Mike Bostock's D3 gallery and I have some questions. Here is the code snippet from an Observable notebook:

Within this code snippet, there is a block of code that doesn't seem to be a function definition but has a return value:

chart = {
    const svg = d3.select(DOM.svg(width, height));

    const g = svg.append("g")
        .selectAll("g")
        .data(bins)
        .join("g");

    // [...]

    return svg.node();
}

I'm curious about the purpose or meaning of the return statement when it is not within a function definition.

javascriptd3.jsobservablehq

Answer №1

Ah, yes, as pointed out by the commentators, this unique syntax belongs specifically to Observable. What you are observing is a cell utilizing a block structure, as explained in the Introduction to Code.

When comparing this with traditional JavaScript, it's akin to an IIFE, but with the added functionality of automatically resolving references to other cells if mentioned. To illustrate in vanilla JavaScript:

chart = (() => {
    const svg = d3.select(DOM.svg(width, height));

    const g = svg.append("g")
        .selectAll("g")
        .data(bins)
        .join("g");

    // [...]

    return svg.node();
})()

In essence, this is essentially what the code compiles to. The syntax is designed to indicate that it executes when referenced values change - for more insight on this, refer to how Observable runs. Unlike an IIFE, a cell within Observable may run multiple times if any referenced elements like generators or Promises undergo changes.

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