Two pretensions and an evaluation in an if statement

Question

Two pretensions and an evaluation in an if statement

Currently, I am delving into the well-optimized code for voronoi implementation created by Mike Bostock (d3js).

I find myself puzzled by the following snippet of code:

if (!(m = (halfedges = cell.halfedges).length)) return;

You can view the full code here: https://github.com/d3/d3-voronoi/blob/master/src/Diagram.js#L87

My confusion arises from the fact that 'halfedges' and 'm' are variables defined in the line after this test. How does this logic function correctly?
What is the intention behind this test? Could it be a safeguard to prevent errors when the function is called with an incorrect type of cell (missing the 'halfedges' property or not being of type array)?

javascript d3.js

Answer 1

Answer №1

Due to the concept of variable hoisting, it is possible for the `var` keyword to be placed anywhere in a function but will still be processed first in its scope:

x = 5;
var x;

// Essentially means:
var x;
x = 5;

It's important to note that the assignment operator `=` is used, not the equality operator `==`. This code snippet involves two assignments and a test on the length of `cell.halfedges`, not a comparison between `m` and `halfedges`.

The code could also be expressed as:

if (!cell.halfedges.length) return;
halfedges = cell.halfedges;
m = halfedges.length;

Answer 2

Due to the concept of variable hoisting, it is possible for the `var` keyword to be placed anywhere in a function but will still be processed first in its scope:

x = 5;
var x;

// Essentially means:
var x;
x = 5;

It's important to note that the assignment operator `=` is used, not the equality operator `==`. This code snippet involves two assignments and a test on the length of `cell.halfedges`, not a comparison between `m` and `halfedges`.

The code could also be expressed as:

if (!cell.halfedges.length) return;
halfedges = cell.halfedges;
m = halfedges.length;

Two pretensions and an evaluation in an if statement

Answer №1

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