Add first example implementation of basic translation

packages/jspsych/src/JsPsych.ts

@@ -86,6 +86,11 @@ export class JsPsych {
   // of jsPsych
   webaudio_context: AudioContext = null;
 
+  /**
+   * the currently active locale (if specified)
+   */
+  locale: string | undefined = undefined;
+
   internal = {
     /**
      * this flag is used to determine whether we are in a scope where
@@ -117,10 +122,15 @@ export class JsPsych {
       override_safe_mode: false,
       case_sensitive_responses: false,
       extensions: [],
+      translations: {},
+      locale: undefined,
+      fallback_locale: undefined,
       ...options,
     };
     this.opts = options;
 
+    this.locale = this.opts.locale;
+
     autoBind(this); // so we can pass JsPsych methods as callbacks and `this` remains the JsPsych instance
 
     this.webaudio_context =
@@ -379,6 +389,49 @@ export class JsPsych {
     return this.timeline.allTimelineVariables();
   }
 
+  setLocale(locale: string) {
+    this.locale = locale;
+  }
+
+  translate(key: string) {
+    // TODO: This should support string processing + maybe more i18n features
+
+    // First pass at translation using the current locale
+    const locale = this.locale;
+    if (locale) {
+      const translation = this.searchTranslation(locale, key);
+      if (translation !== undefined) {
+        return translation;
+      }
+    }
+    // Second pass using the fallback_locale
+    const fallbackLocale = this.opts.fallback_locale;
+    if (fallbackLocale) {
+      const translation = this.searchTranslation(fallbackLocale, key);
+      if (translation !== undefined) {
+        return translation;
+      }
+    }
+    // If nothing is found, default to return the original translation key
+    return key;
+  }
+
+  private searchTranslation(locale: string, key: string) {
+    // First search for translations on the trial itself
+    if (
+      this.current_trial.translations &&
+      locale in this.current_trial.translations &&
+      key in this.current_trial.translations[locale]
+    ) {
+      return this.current_trial.translations[locale][key];
+    }
+    // Then search for global translations
+    if (locale in this.opts.translations && key in this.opts.translations?.[locale]) {
+      return this.opts.translations[locale][key];
+    }
+    return undefined;
+  }
+
   addNodeToEndOfTimeline(new_timeline, preload_callback?) {
     this.timeline.insert(new_timeline);
   }

packages/jspsych/tests/core/i18n.test.ts

@@ -0,0 +1,73 @@
+import htmlKeyboardResponse from "@jspsych/plugin-html-keyboard-response";
+import { pressKey, startTimeline } from "@jspsych/test-utils";
+
+import { initJsPsych } from "../../src";
+
+describe("translations are correctly fetched", () => {
+  test("when drawing from the global translations", async () => {
+    const jsPsych = initJsPsych({
+      locale: "de",
+      translations: {
+        de: {
+          "Hello World!": "Hallo Welt!",
+        },
+      },
+    });
+    const { getHTML } = await startTimeline(
+      [
+        {
+          type: htmlKeyboardResponse,
+          stimulus: () => jsPsych.translate("Hello World!"),
+          on_finish: () => {
+            jsPsych.setLocale("en");
+          },
+        },
+        {
+          type: htmlKeyboardResponse,
+          stimulus: () => jsPsych.translate("Hello World!"),
+        },
+      ],
+      jsPsych
+    );
+
+    expect(getHTML()).toMatch("Hallo Welt!");
+    pressKey("a");
+    expect(getHTML()).toMatch("Hello World!");
+  });
+
+  test("when drawing from trial-level translations", async () => {
+    const jsPsych = initJsPsych({
+      locale: "de",
+    });
+    const { getHTML } = await startTimeline(
+      [
+        {
+          type: htmlKeyboardResponse,
+          stimulus: () => jsPsych.translate("Hello World!"),
+          translations: {
+            de: {
+              "Hello World!": "Hallo Welt!",
+            },
+          },
+          on_finish: () => {
+            jsPsych.setLocale("en");
+          },
+        },
+        {
+          type: htmlKeyboardResponse,
+          stimulus: () => jsPsych.translate("Hello World!"),
+          translations: {
+            de: {
+              "Hello World!": "Hallo Welt!",
+            },
+          },
+        },
+      ],
+      jsPsych
+    );
+
+    expect(getHTML()).toMatch("Hallo Welt!");
+    pressKey("a");
+    expect(getHTML()).toMatch("Hello World!");
+  });
+});