codeframe.js
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/* @flow */
const range = 2
export function generateCodeFrame (
source: string,
start: number = 0,
end: number = source.length
): string {
const lines = source.split(/\r?\n/)
let count = 0
const res = []
for (let i = 0; i < lines.length; i++) {
count += lines[i].length + 1
if (count >= start) {
for (let j = i - range; j <= i + range || end > count; j++) {
if (j < 0 || j >= lines.length) continue
res.push(`${j + 1}${repeat(` `, 3 - String(j + 1).length)}| ${lines[j]}`)
const lineLength = lines[j].length
if (j === i) {
// push underline
const pad = start - (count - lineLength) + 1
const length = end > count ? lineLength - pad : end - start
res.push(` | ` + repeat(` `, pad) + repeat(`^`, length))
} else if (j > i) {
if (end > count) {
const length = Math.min(end - count, lineLength)
res.push(` | ` + repeat(`^`, length))
}
count += lineLength + 1
}
}
break
}
}
return res.join('\n')
}
function repeat (str, n) {
let result = ''
if (n > 0) {
while (true) { // eslint-disable-line
if (n & 1) result += str
n >>>= 1
if (n <= 0) break
str += str
}
}
return result
}