Add custom inspect for node without depedency (#1526)

src/datetime.js

@@ -1935,6 +1935,18 @@ export default class DateTime {
     return this.isValid ? this.toISO() : INVALID;
   }
 
+  /**
+   * Returns a string representation of this DateTime appropriate for the REPL.
+   * @return {string}
+   */
+  [Symbol.for("nodejs.util.inspect.custom")]() {
+    if (this.isValid) {
+      return `DateTime { ts: ${this.toISO()}, zone: ${this.zone.name}, locale: ${this.locale} }`;
+    } else {
+      return `DateTime { Invalid, reason: ${this.invalidReason} }`;
+    }
+  }
+
   /**
    * Returns the epoch milliseconds of this DateTime. Alias of {@link DateTime#toMillis}
    * @return {number}

src/duration.js

@@ -608,6 +608,18 @@ export default class Duration {
     return this.toISO();
   }
 
+  /**
+   * Returns a string representation of this Duration appropriate for the REPL.
+   * @return {string}
+   */
+  [Symbol.for("nodejs.util.inspect.custom")]() {
+    if (this.isValid) {
+      return `Duration { values: ${JSON.stringify(this.values)} }`;
+    } else {
+      return `Duration { Invalid, reason: ${this.invalidReason} }`;
+    }
+  }
+
   /**
    * Returns an milliseconds value of this Duration.
    * @return {number}

src/interval.js

@@ -539,6 +539,18 @@ export default class Interval {
     return `[${this.s.toISO()} – ${this.e.toISO()})`;
   }
 
+  /**
+   * Returns a string representation of this Interval appropriate for the REPL.
+   * @return {string}
+   */
+  [Symbol.for("nodejs.util.inspect.custom")]() {
+    if (this.isValid) {
+      return `Interval { start: ${this.s.toISO()}, end: ${this.e.toISO()} }`;
+    } else {
+      return `Interval { Invalid, reason: ${this.invalidReason} }`;
+    }
+  }
+
   /**
    * Returns a localized string representing this Interval. Accepts the same options as the
    * Intl.DateTimeFormat constructor and any presets defined by Luxon, such as