wx.arrayBufferToBase64 方法在小程序>微信小程序的基础库版本 2.4.0 起已废弃，不建议继续使用。
我们可以将ArrayBuffer 转换为 base64，使用base64-js来实现转换。具体操作如下：
1、安装base64-js,可以通过npm进行安装，或者从github获取该代码块。 代码如下：

(function (a) {if ("object" == typeof exports && "undefined" != typeof module) module.exports = a();else if ("function" == typeof define && define.amd) define([], a);else {var b;b = "undefined" == typeof window ? "undefined" == typeof global ? "undefined" == typeof self ? this : self : global : window, b.base64js = a()}
})(function () {return function () {function b(d, e, g) {function a(j, i) {if (!e[j]) {if (!d[j]) {var f = "function" == typeof require && require;if (!i && f) return f(j, !0);if (h) return h(j, !0);var c = new Error("Cannot find module '" + j + "'");throw c.code = "MODULE_NOT_FOUND", c}var k = e[j] = {exports: {}};d[j][0].call(k.exports, function (b) {var c = d[j][1][b];return a(c || b)}, k, k.exports, b, d, e, g)}return e[j].exports}for (var h = "function" == typeof require && require, c = 0; c < g.length; c++) a(g[c]);return a}return b}()({"/": [function (a, b, c) {'use strict';function d(a) {var b = a.length;if (0 < b % 4) throw new Error("Invalid string. Length must be a multiple of 4");var c = a.indexOf("="); - 1 === c && (c = b);var d = c === b ? 0 : 4 - c % 4;return [c, d]}function e(a, b, c) {return 3 * (b + c) / 4 - c}function f(a) {var b, c, f = d(a),g = f[0],h = f[1],j = new m(e(a, g, h)),k = 0,n = 0 < h ? g - 4 : g;for (c = 0; c < n; c += 4) b = l[a.charCodeAt(c)] << 18 | l[a.charCodeAt(c + 1)] << 12 | l[a.charCodeAt(c + 2)] << 6 | l[a.charCodeAt(c + 3)], j[k++] = 255 & b >> 16, j[k++] = 255 & b >> 8, j[k++] = 255 & b;return 2 === h && (b = l[a.charCodeAt(c)] << 2 | l[a.charCodeAt(c + 1)] >> 4, j[k++] = 255 & b), 1 === h && (b = l[a.charCodeAt(c)] << 10 | l[a.charCodeAt(c + 1)] << 4 | l[a.charCodeAt(c + 2)] >> 2, j[k++] = 255 & b >> 8, j[k++] = 255 & b), j}function g(a) {return k[63 & a >> 18] + k[63 & a >> 12] + k[63 & a >> 6] + k[63 & a]}function h(a, b, c) {for (var d, e = [], f = b; f < c; f += 3) d = (16711680 & a[f] << 16) + (65280 & a[f + 1] << 8) + (255 & a[f + 2]), e.push(g(d));return e.join("")}function j(a) {for (var b, c = a.length, d = c % 3, e = [], f = 16383, g = 0, j = c - d; g < j; g += f) e.push(h(a, g, g + f > j ? j : g + f));return 1 === d ? (b = a[c - 1], e.push(k[b >> 2] + k[63 & b << 4] + "==")) : 2 === d && (b = (a[c - 2] << 8) + a[c - 1], e.push(k[b >> 10] + k[63 & b >> 4] + k[63 & b << 2] + "=")), e.join("")}c.byteLength = function (a) {var b = d(a),c = b[0],e = b[1];return 3 * (c + e) / 4 - e}, c.toByteArray = f, c.fromByteArray = j;for (var k = [], l = [], m = "undefined" == typeof Uint8Array ? Array : Uint8Array, n = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/", o = 0, p = n.length; o < p; ++o) k[o] = n[o], l[n.charCodeAt(o)] = o;l[45] = 62, l[95] = 63}, {}]}, {}, [])("/")
});

2、在utils文件下，建一个base64.js。将base64-js的代码复制到base64.js内。
3、在utils文件下，建一个util.js。引入base64.js文件。

import base64 from "@/utils/base64.js"
const toBase = (arrayBuffer) => {const uint8Array = new Uint8Array(arrayBuffer);const base64Data = base64.fromByteArray(uint8Array);return base64Data;
}
export {toBase 
}

4、页面使用此方法

import { toBase } from '@/utils/util.js'
//转换图片 需要加上'data:image/png;base64,'   
//不需要转则直接toBase(接口返回的文件流)
const base64Data = 'data:image/png;base64,' + toBase (接口返回的文件流);