postcss-advanced-variables/lib/get-closest-variable.js

// get the closest variable from a node
export default function getClosestVariable(name, node, opts) {
	const variables = getVariables(node);

	let variable = variables[name];

	if (requiresAncestorVariable(variable, node)) {
		variable = getClosestVariable(name, node.parent, opts);
	}

	if (requiresFnVariable(variable, opts)) {
		variable = getFnVariable(name, node, opts.variables);
	}

	return variable;
}

// return the variables object of a node
const getVariables = node => Object(Object(node).variables);

// return whether the variable should be replaced using an ancestor variable
const requiresAncestorVariable = (variable, node) => undefined === variable && node && node.parent;

// return whether variable should be replaced using a variables function
const requiresFnVariable = (value, opts) => value === undefined && Object(opts).variables === Object(Object(opts).variables);

// return whether variable should be replaced using a variables function
const getFnVariable = (name, node, variables) => 'function' === typeof variables
	? variables(name, node)
: variables[name];