Javascript closures do not correctly access external variables when using D3.js?

d3.js v5 version is in use, as follows: javascript code snippet

let colorData = [1, 2, 3, 4, 5, 6, 7, 8];
let colorSchemes = [
    d3.scaleOrdinal(d3.schemeCategory10),
    d3.scaleOrdinal(d3.schemeAccent),
    d3.scaleOrdinal(d3.schemePaired),
    d3.scaleOrdinal(d3.schemePastel1),
    d3.scaleOrdinal(d3.schemeDark2),
    d3.scaleOrdinal(d3.schemeSet1)
];

let scheme = colorSchemes[0];
let test_color = scheme(1); // test_color = -sharp1f77b4

// here ignore some code about "svgs"
svgs.selectAll("circle")
    .data(colorData).enter()
    .append("circle")
    .attr("fill", (d, i) => {
        let scheme = colorSchemes[i];
        return scheme(d); // the console log "scheme is not a function"
    });

Why does test_color get the value correctly while the same code in the closure does not execute correctly?

D3.js javascript

Mar.02,2021

answer your own questions.
it turns out that I misunderstood the meaning of the variable I in the above code. I is the index number of colorData, not the index number of colorSchemes.
the following corrected code:

let colorData = [1, 2, 3, 4, 5, 6, 7, 8];
let colorSchemes = [
    d3.scaleOrdinal(d3.schemeCategory10),
    d3.scaleOrdinal(d3.schemeAccent),
    d3.scaleOrdinal(d3.schemePaired),
    d3.scaleOrdinal(d3.schemePastel1),
    d3.scaleOrdinal(d3.schemeDark2),
    d3.scaleOrdinal(d3.schemeSet1)
];

// here ignore some code about "svgs"
svgs.each(function (d, schemeIndex) {
    d3.select(this)
        .selectAll("circle")
        .data(colorData)
        .enter().append("circle")
        .attr("fill", (d, i) => {
            let scheme = colorSchemes[schemeIndex];
            return scheme(d);
        });
});

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